Reserve the least expensive modelOnce you select a model, no rental-car company will ever let you leave with a cheaper one. When the car you reserved isn't available, they'd rather give you a better car for the same price than a lesser car for a lower price. If you're inclined, you can negotiate an upgrade at the car-rental counter.
Do ask, do tell
Laws vary by state and rules vary by company, but unmarried couples can often skip the extra-driver fee (and the paperwork) simply by telling the clerk that they're domestic partners. Don't wait for the clerk to ask what your relationship is. Other ways to try to get around the extra-driver fee: joining the loyalty club (what on earth are you waiting for?), saying that you're coworkers or that you're members of AAA, AARP, or Costco. The point is to ask.
Grab the keys and go
Join the free loyalty club and you won't have to wait in what can be a cruelly long line, or tell the agent your address, or listen to the upsell on a bigger model or on insurance. You can get off the lot and into your vacation in a matter of minutes.
Think globally, browse locally
It's a little-known fact that some car-rental companies have different websites for different countries, and the rates for the same rental can vary. The prices quoted at Budget.com, for instance, won't necessarily be equal to the rates at the sites for the United Kingdom (budget.co.uk), Germany (budget.de), Ireland (budget.ie), and South Africa (budget.co.za). It's worth poking around.
Don't pay double for insurance
Insurance is a big profit generator for car-rental agencies, and they don't mind that it's confusing. If you own a car, however, your owner's policy will cover you no matter what car you're driving, including a rental. Also, most credit cards cover the Collision Damage Waiver (CDW) automatically, so there's no need to give the rental agency that extra $10 to $20 per day.