My two solutions:
1) Pay 'em. All of them. Cut them a check for $25k right before training camp. But, they're responsible for all of their own bills - tuition, rent, bills, food, booze, personal trainers, coaching instruction, etc. 95% of them would be broke by October 1st.
2) Get rid of the rule that says they have to go to college. I'd love to see Manziel flounder through an NFL training camp at 18 years old. Very few of these players could make the immediate jump. They need the system, and they'd figure that out pretty damn quick.