1. Compare prices at Kayak.com.

2. Book your rental car--at a refundable rate--as early as possible. Then check prices again 1-2 weeks before your trip, and again the day before. Agencies tend to run specials when they have a lower than expected rental rate and you can take advantage of that by rebooking at the special rate. That's why you want to book your first rental as a refundable reservation.

3. Check priceline.com after step 2; you might be able to snag an even better deal. Bid low; you can always bid again if your bid is rejected. Priceline rentals are non-refundable, so no more price checking after this!

4. Decline extra insurance. American Express covers insurance for car rentals, and some car policies do, too. If there is an extra charge for a second driver, take turns and have just one driver each vacation.

5. Bring your own GPS, toll pass and car seat so you don't have to pay extra to rent these.

6. Beware pre-pay gas options. The way this works, you buy the first tank from the rental company, and bring it back (ideally, for you) on fumes. But the pre-pay price can be higher than at local gas stations, and if you fill the tank and don’t use all the gas, you are essentially paying for the gas twice.

7. The biggest way to save is to cut down on the number of days of the car rental. Last year, we had a late flight into Florida over the holidays and stayed at an airport hotel the first night. We took the free shuttle to the hotel, then took the free shuttle back to the airport the next day to pick up the car, saving $100 on that day's rental.

8. Rent from non-airport locations. When Nasreen Stump, Traveling with Babies Mom, visits Seattle she takes the bus into town and rents from a downtown location. She notes that “airports charge all kinds of additional fees. Often you can rent from a downtown location and return to the local airport for no additional charge and not pay airport fees.”

9. Be nice. Ellen Seidman, SpecialKid TravelingMom and Love That Max, says, "We always ask for an upgrade, and 80 percent of the time we get one. Sometimes, it really is as simple as asking nicely.”

10. Forego frequent flier tie-ins.  You might get charged a convenience fee if the rental car agency has to communicate with another loyalty program about points.

11. Finally, drive safely, avoid penalties and return the car on time. All your savings will go out the window if you nick the car or return it late.

Click here for tips on renting a car in Europe.