RULE NO. 1: Unless the server is rude, condescending, and/or completely absent, tip between 18 and 20 percent.
RULE NO. 2: Never tip on tax. Tip based on the subtotal. And if you're calculating your tip simply by doubling the tax, stop it--you're being cheap.
RULE NO. 3: Unless you drink like Dean Martin or have a taste for expensive wines (i.e., $40 or more, depending on your budget), it's best to include booze when calculating a tip. Bartenders expect a dollar tip per drink (which is usually about 20 percent of the drink's price), and it's no different with waiters.
RULE NO. 4: Never turn a blind eye when others are tipping--especially if they're unfamiliar with our tipping culture (i.e., Europeans). If you think your tablemate is lowballing the service, it's best to hand the waiter a few bills on the way out.
RULE NO. 5: If a few dollars here and there really matter that much to your bank account, perhaps you shouldn't be going out to eat in the first place.