Fake it til you make it is valid from the standpoint of brain science. I'll explain from what I can remember, since I'm still at the "dabbling" stage of knowledge. I encourage you to get out there and study it for yourself if you're interested.
The conscious mind is only one sixth of the brain. The other five sixths is the non-conscious brain. The non-conscious brain sees in images, and takes everything as fact. Examples of this include crying at a sad movie, or feeling frightened during a horror movie. Your rational conscious mind knows that it's only a film, and that it's not real. But the non-conscious mind believes what it sees, period.
If you think about that fact for a minute or two, no further explanation is needed, though I'll try to give it since I'm bored. "Fake it til you make it" is essentially using your conscious mind to reprogram your non-conscious mind. You're not attempting to deceive yourself.
What can be gained by reprogramming the non-conscious mind? Well, for starters the non-conscious mind is a lot bigger than the conscious one. Imagine pitting a scrawny 100 pound man against a 500 pound gorilla in a boxing ring. Who's gonna win? The impulses and thoughts coming up from the non-conscious mind are going to be very hard for the conscious mind to resist. So, use that to your advantage.
It's important to understand that you're not actually attempting to delude your conscious mind. For example, say you want a healthier self image. You look in the mirror every morning and announce "I look fantastic today!"... and your conscious mind is going to pitch a fit. What the hell? You look like utter crap. Don't you see *insert flaw here*? Now not only are you one ugly bastard, you're a liar on top of it. You actually believe in that affirmation junk? Great, now you're stupid on top of everything else. etc, etc. But since you're not talking to the conscious mind, just ignore it.
This is the science behind affirmations. The Secret is a load of bull (no offense to anyone who believes in it). They say that the affirmations you speak on a daily basis will trigger the power of the universe, and that the universe will give things to you... etc. No, that's incorrect. The affirmations work because they retrain the automatic thought patterns of the non-conscious mind, and the conscious mind is almost completely influenced by the thoughts that rise up from the non-conscious portion of the brain. So if you're using affirmations for wealth and results start appearing, you're not attracting wealth from the bounty of the universe. Your non-conscious mind has simply been told to look for opportunities for the expansion of wealth and it's been more receptive to such things.
Negative positives are not your achievement friend. An example of an affirmation fail is "Don't overeat." If I say "Don't imagine a pink elephant!", what happens? You see it, don't you? Okay, apply that to "Don't overeat." The image triggered was that of what you DO NOT WANT... and it was an order to the non-conscious mind (remember, it believes pictures), isn't that sad?
"Don't screw up this meeting."
"Don't say the wrong thing."
"Don't procrastinate."
"Don't lose this game."
Etc.
A positive positive is the correct format for the non-conscious mind, since it generates an image of what you actually wish the achieve. It takes conscious effort to change your self talk from negative to positive.
I could go on and on and on. But to answer the question, yes, fake it til you make it works... if done correctly. And having a grasp of the science behind what you're doing helps tremendously.