The common view of free will among both religious believers and nonbelievers is that materialism and determinism contradict free will.  To summarize Sam Harris' position, your choices are merely your genetic predispositions reacting to your environment.  You can't choose your genetics, and you can't choose your upbringing and other outside influences, so ultimately you can't choose your destiny.

(See Steven Pinker's video lecture Chalking it up to the blank slate for more information and case studies illustrating the importance of genetics.  I'll talk about other types of influences later in this article.)

A result of this philosophy is there's no legitimate reason to be proud of anything, since the factors leading to success are all outside of the individual's control.  Not only is the predisposition to have a talent genetic, but there's evidence that even the inclination to work hard and develop that talent is genetic.  Perhaps you were born with the laziness gene, but you were raised to value hard work and overcame it – but you couldn't choose your upbringing any more than you could choose your genetics, so it's all still circumstantial.

Was there any point in including the previous paragraph?  If you have an OXTR gene with a guanine allele, you'll probably ahead and be proud anyway – unless reading the previous paragraph was enough of an environmental influence to override the gene.

To avoid this ostensibly dismal predestination, religious people posit an immaterial soul that defines who we are.  With that in mind, let's examine another example.  Imagine that you were born with the MAOA "warrior" gene, and you grew up in an environment that encouraged aggression, and yet through a highly unlikely twist you ended up a pacifist.  What made you different from everyone else with the genetics and upbringing for violence?  God made your soul more peaceful than the others?  Metaphysical allure notwithstanding, that would merely another predisposition beyond your control.  The existence of a soul would add another variable, but it wouldn't change the fundamental nature of the problem.

Now let's reexamine our definition of free will.  The assumed definition has been something to the effect of "the ability to have done otherwise independently of internal factors (i.e. intrinsic genetic nature) and external factors (i.e. life experience)."  If neither internal nor external motivations count as free will, what the hell would?

The problem here isn't that we don't have free will; it's that we've been defining it in a way that's logically absurd.  So given a suitable definition, do we have free will?  You decide…
We definitely have will: impulses, desires, and goals.  We are free to act according to our will.  This is what we experience as "free will" when we're not contemplating philosophy, which is why it's nigh impossible not to believe we have it.  But the story doesn't end there.

While we are free to make decisions based on our will, we can't control what our will is in the first place.  We can neither turn off our hunger nor can we simply stop wanting to be skinny.  Even if we could, which inclinations we would elect to deactivate would just be based on other non-elective inclinations, bringing us right back to where we started.

As I wrote earlier, the fashionable conclusion is that all motivating factors subvert free will, and therefore our sense of freedom is illusory.  However, defining free will as a concept that's not even hypothetically imaginable and disproving it is a mathematician's answer, no more satisfying than disproving the existence of a four-sided triangle or the notorious Invisible Pink Unicorn.  To evaluate the matter in a way that's meaningful, we'll have to establish what sort of motivating factors would be in accordance with free will, and which ones would preclude it.  Let's examine a few questionable examples…

Brain Slug Mind Control: A nefarious individual puts a chip in your brain which affords them direct control of neurochemical levels and neural activity.  By remote control, they can govern how you feel and which options seem appealing to you.  Since they're controlling the subjective feelings associated with your decisions, and not forcing your actions directly, the illusion of freedom is maintained.  I don't think anyone would call this free will, unless they already had a chip in their brain making them say it.

You might think I'm making this up, but here's a video of someone using a prototype of such a device (starting at 1:45).

Cult-Style Brainwashing: Just what it says on the tin.  A prime example of a brainwashing cult was the Seekers.  They prophesized an exact date for doomsday, when a celestial incarnation of Jesus would arrive and rescue them.  When the date arrived and nothing happened, they were so resolute in their unfounded belief that they immediately devised a rationalization to protect the belief, without even considering the possibility that the prophesy was simply untrue.  A more famous and recent example was David Koresh's Branch Davidian cult in Waco, Texas.  Did Koresh's subjects choose to follow him of their own volition, or did he override their free will?

Deceptive Manipulation: The best example I can give here is marketing.  However underhanded you think it is, it's worse.  The links below provide a few examples.

The Decoy Effect

The Unit Effect

The Charlie's Angels Effect

Derren Brown's "Subliminal Messages" trick.  (This other video of it includes different details in the final explanation.)  This one isn't exactly a marketing trick, but it's related.  It uses subliminal messages to influence advertising experts to unwittingly create a drawing of his choice.  In this particular trick he uses environmental cues, but in other similar tricks he uses body language, tone, and word choice to subtly influence participants.  For example, to make someone draw a certain person sight unseen, he might wave his hands over his head in a way that subconsciously reminds the participant of long hair, or work words like 'bland' or 'blind' into the conversation to covertly suggest 'blonde'.

However, not all manipulations are to one's detriment.  Parents may use child-rearing techniques to instill desirable attributes into their offspring.  If you're being deliberately manipulated without your knowledge, but it's to your advantage and you'd go along with it if you knew, does that still count as an infringement on your free will?  Does covert manipulation nullify free will regardless of the outcome?

Unintentional and Indirect Influences:  Even when nobody is purposely manipulating you, you can still be affected by your environment and other factors.  When there isn't another will with an ulterior motive behind it, the coercion seems like less of a violation of free will – but is that fair to say?

The most familiar and prevalent example I can provide is religion.  (Although religious indoctrination is deliberate in a lot of ways, its omnipresence makes it primarily an environmental factor.)  The influence is so powerful that a given person is almost certain to adopt the religion of those around them and see any other religious belief as unthinkable.  (The video series in the previous link goes beyond religion-by-location and delves into the psychological and evolutionary mechanisms that fuel religion.  I highly recommend it.)  Other more subtle examples are linked below.

A Million Deaths is a Statistic

Video: Self Skepticism

Video: How upbringing affects critical thinking and other attributes

Some of the examples shown are cognitive illusions that cause us to make regrettably illogical decisions that we wouldn't make with the proper reflection.  Is this our innate nature (which we couldn't choose) betraying us behind the scenes, giving us impulses to act irrationally while fooling us into thinking we're doing the right thing, or is it us using our free will to decide poorly?

No Influence at All:  I only include this one to highlight that it's impossible.  Imagine that one grew up under an exotic, inconceivable set of circumstances in which they learned language and other life skills in a cultural vacuum, without ever being exposed to the media or any social interaction.  That would be a distinction to which one might attribute their thoughts and actions.

Why is he so socially awkward?  He grew up alone.  Why does he value education over pop culture entertainment?  He was raised without public media.  Why does he place so little worth on material possessions and conspicuous consumption?  The media never taught him to favor luxury brands and admire wealth.  Why does he not worship our god(s)?  He was never indoctrinated into any religion.

Anosognosia:  Anosognosia is the condition of being unaware of one's disability even after it's been demonstrated.  An example given in this video is someone with left-arm paralysis.  If asked to raise their right arm they'll comply without hesitation, but if asked to raise their left, they'll agree to move it, but then they won't actually do it.  Shockingly, when asked why they didn't move, they won't admit to the experimenters or even themselves that they couldn't.  Instead they'll invent an ad hoc excuse that implies that they freely chose not to move it (or that an outside factor was responsible for the failure, and not a disability of theirs).

The video goes on to explain that there's a separate brain function responsible for monitoring the presence of motor function and sensory input, and when that malfunctions, the speech and rationalization centers of the brain take over the role.  With rationalization as an automatic and involuntary backup, someone can be convinced that they're making deliberate choices when they're obviously not.

Neither link brings up free will, but the implications are startling.  If the faulty "wiring" in these patients' brains can create the facade of free choice that is clearly illusory, then who's to say we aren't experiencing something very similar?  The brain chemicals and neuron signal firings really call all the shots, and reassuring us that we are in control is all just part of the mechanism.  Only when the elaborate biological clockwork glitches does this unsettling reality come to light.

This is not to be dismissed as an embellishment of an obscure phenomenon.  If I were to ask you why you made a certain decision, you could almost certainly produce reasons.  But how often do you make a decision because it just seems right, and then come up with justifications after the fact?  How can you be so sure that the reasons you gave were really what motivated you, and not other factors of which you weren't consciously aware?

Still, free will isn't an open and shut case.  A strong case can be made that you simply are your brain (or more precisely, that your mind is an emergent property of your brain, a process, and not a separate entity).  In this case, your brain performing operations without your conscious knowledge or consent isn't a breach of free will from a malignant factor outside of yourself, but it does lead to some paradoxical accounts of covert self-deceit and self-conflict.

Even if a spiritual soul were proven to exist (not likely; see chapters 10-12 of The Best Case for details), studies of neurological mishaps show us that the "soul" couldn't be the one in control anyway, so it's really "immaterial".

Dan Dennett goes into more detail about the idea of the mind being composed of many distinct parts that (usually) work in concert, and why proposing a central consciousness in charge of the individual neural processes merely presents a bigger mystery, in his video lecture The Magic of Consciousness.

Determinism:  Determinism isn't an influence per se, but it's a common view that determinism is the final nail in the coffin of free will, so I wanted to take a moment to address that.  The neurological and deterministic case against free will holds that if one knows enough about the states and activities of your brain, they can know, with absolute certainty, what you will decide even before you do.

This sounds alarming, until you realize that somebody who simply knows you well enough can reliably predict your decisions, and that only requires the mundane means of personal experience.  Furthermore, if I give you the option of either a hundred dollars or an autographed copy of this essay, I can foresee your decision without even knowing you.  (I know my writing isn't that amazing, so I forgive you in advance for taking the Benjamin.)  In fact, the only way to avoid being predictable is to decide randomly, and randomness certainly isn't free will.  To summarize my position on free will versus determinism, predicting the weather isn't the same as controlling it.

My conclusion is this: the question of free will is a semantic dispute since there's a general consensus on what's essentially going on; it merely depends on where you draw the line.  Instead of questioning whether our choices are really our own in a distant, philosophical sense, I advocate researching our own innate biases and heuristics, and others' manipulation tactics.  Understanding and correcting for our own evolutionary flaws will give free will the best fighting chance.