1. When making a rental car reservation, particularly a short one, check out the cost of renting for a longer time. Although it's counter-intuitive, the longer rental might cost less than the shorter one. This is because rental car companies are prone to sock-it-to less-price-concerned business travelers (on reimbursed expense budgets) who tend to do shorter term rentals.
2. Although you might be tempted to take the relatively small discount rental car companies offer to pay in full when making your reservation, avoid the temptation, particularly for rentals being made far ahead of the time needed. This is because it deprives you of the ability to continue price-checking and then cancelling your existing rental and switching to a different rental if you find a better price.