Here's a useful tip from Buget Travel Magazine. I always found that planning ahead and shopping online on car rentals can definitely save you money. Here's a way to try to get a free upgrade..
How: Get a free upgradeBook the cheapest rental car you can live with and arrive early in the day, before most customers have returned their cars. Result? The agency may not have any cheap cars to give you, and they are obligated to give you a bigger model at no extra charge. If they try to sell you an upgrade, say no. If it turns out they haven't run out of cheap cars, politely ask for a free upgrade.
Read more: http://www.budgettravel.com/feature/budget-travel-advice-save-money-on-summer-travel,8696/#ixzz24zcTQeXr