In no particular order (other than what I can remember):
Make sure your tires are properly inflated. (35psi Front, 33psi Rear, MINIMUM)
Have your car serviced according to schedule, and make sure your dealer uses 5W30 oil.
If you're carrying something in the car that you really don't need to, remove it. (the common example is the golf clubs in the trunk...) extra weight will reduce mileage.
Anticipate traffic. coast to a stop rather than suddenly slamming on your brakes when you meet up with the stopped traffic.
Stay within the posted speed limits. Particularly at highway/freeway speeds, going faster will increase your wind resistance, which means more work for your engine which equals lower MPG.
Roof rack or trunk rack for luggage/bicycles/skis: when not using it, remove it. If at all possible, put the mounted item in the car rather than mounting it outside. The added rack will add wind resistance (and also change your aerodynamics) and lower your MPG.
Clean the snow off of your car before you drive. Not only is the snow flying off of your car a nuisance/hazard to other drivers, but the snow changes the aerodynamics of the car (creating more wind resistance) and also adds weight to your car, lowering MPG.
Don't use high-octane fuel. Stick to the "regular" 87 octane that the Prius was designed for. Higher octane fuel is just more expensive, doesn't increase MPG (may even lower it due to less energy available in higher octane fuel), and has been known to give the Prius a check-engine light.