53 MPG is not bad at all. Here are some tips that helped me improve my gas mileage from 43.3 MPG to 62.1 MPG.
1. Don't go over 65 MPH on the freeway. The fuel efficiency drops drastically between 65 and 70 MPH. You will get the best gas mileage at 55 - 59 MPH.
Poor advise. Don't EVER go below the stated speed limit. I recently took a course on driver safety and the damn instructor wanted to spend 30 minutes chastising drivers for going over the limit. All the while I'm looking at his handout that shows drivers who were UNDER the limit/driving too slow, were the NUMBER 1 CAUSE OF ALL ACCIDENTS, BAR NONE. I repeat, going below the speed limit is the number 1 cause of all accidents on the road today. Keep up with the flow of traffic. Don't forgo safety for mpg. If traffic is too fast for you, get off the interstate and take a different route. It's safer.
And, from personal experience I've found the largest factor in determining gas mileage in a Prius is the use of climate controls. The fan in particular. Try setting your temp, then put fan on lowest setting. If you need to cool/warm up quickly, set fan a little higher, then down to lowest setting after a few minutes.