The market. Right now, the government loan programs treat all degrees interchangeably. If you introduce market forces into the equation, the lenders will run some basic stat calculations and you'll quickly end up with the following scenario:

1. Useful degrees from good universities will be easy and cheap for good students to fund.

2. Everything else will get harder/more expensive to fund, in proportion to its statistical returns.

That's all you really need to fix the system.