If I'm given the option of selecting my own rental car, I like to choose one that does NOT have Florida plates on it (when possible). By doing so, it immediately eliminates about 75% of the cars in parking lots.
Don't buy the insurance that they will try to sell you. You don't need it and it will dramatically raise the price of your rental.
Look early and often for car rentals. As long as you don't prepay, you can book and cancel as many as you want. I just make a new reservation for my mom's December trip last night. It had dropped $20. She asked me to change the pick up time to one hour earlier (it shouldn't matter but I do it to please her) and the price dropped another $15.
IMO, don't both renting a GPS. Orlando has great signage to direct you to both Disney and Orlando. IF you have your own, bring that if you think you'll want one.