Step 1: Tires
Should you wish to go beyond, that is up to you. Personally, I inflate my tires to around 55psi (they are rated for 44). Please do not attempt on old or visibly damaged tires. My tires are new and in excellent condition.
Modern tires are radially belted - that is, they won't balloon like tires of yore. Keep in mind what you're doing though. You are reducing the contact patch of your tires in order to decrease static friction. This will reduce your overall braking ability. So riding someone's bumper at 80mph is just that much more dangerous. Again, these steps only work when combined - you can't pick and choose.
When it comes time to buy new tires, look for low rolling resistance (RR) tires. While they may not have the same 60,000 mile (or even 40,000) guarantee, they will save you in the fuel department. The Toyota Prius comes equipped with LRR tires.
Step 2: Weight
For those more daring, remove your emergency jack and spare tire and be sure to carry a cell phone in case of emergency. AAA is a good thing to invest in - even if you don't plan on following these mileage techniques.
If you won't be carrying passengers, remove your rear seat and passenger seat as this will lighten your load by a couple hundred pounds.