That's the million dollar question. If anyone knew the true secret to velocity, someone would be very wealthy, and everyone would throw 105 MPH!
I believe velocity is like IQ or musical ability. You're born with a genetic base and can improve on that base with hard work, to a degree. But if you're average intelligence, you're never going to be Einstein. And if you have average musical ability, you're never going to be Mozart. Likewise, if you're R.A. Dickey, you're never going to be Aroldis Chapman.
That said, why don't you post here some videos of you pitching and perhaps we can spot deficiencies in your delivery that could be hindering your velocity? Being stuck at 75-77 MPH from 15 to 17 years of age sounds like a mechanical problem.