First, determine how many days you'll want the rental car. Some destinations may not require a car the entire time. Cutting down on rental time can help you save.
When booking your trip, you're likely looking through some of these sites anyway. While you're there, check out their rental car deals.
Another newer service that has some up is Turo. Turo allows you to rent cars from private owners who maybe have an extra vehicle and are trying to make some extra money on the side.