We just returned from our WDW trip, and used a Dollar car for 2 weeks.
The best tip I have is to go to their internet site and sign up for Fastlane. It's free, and it's just a card they give you that has all of your information on it along with your credit card number you plan to use. Once you get the card, you call them and tell them your reservation number, and they will transfer it over to the Fastlane desk.
When you arrive at the airport, you don't have to wait in a long line, just go straight to their Fastlane counter and they will have your car waiting for you! It only took us about 5 minutes!
Oh yeah - the best part is when you sign up, they send you a coupon in the mail for a fee upgrade (you can use it this trip if you want to!)
Hope this helps.