Ask for a receipt each time you go out to drink. Creating a tab at the beginning of the night can help you forget just how much you've been drinking. If you pay for each drink separately, you may have an easier time remembering how much you've had and when you had it. Make sure to tip the bartender plenty, though, especially if you are using a credit card each time.
Buy your own drinks. It can be hard to remember what you've had if other people are buying drinks for you. Thank the other person for the gesture, but have her buy you an appetizer or have a good conversation instead.
Track your consumption with jewelry. Keep a stock of rubber bands or bracelets in your pocket or purse, and put one on each time you have a drink. By the end of the night, it will be obvious how much you've had.
Limit your cash. By only bringing a certain amount of money, you can avoid buying more drinks or food than you intended. If you can, leave your credit cards at home as well.
Order just one drink or none at all. If you're not confident in your ability to limit yourself, be the designated driver. As long as you have your one drink at the beginning of the night, you will likely be sober enough to drive everyone home safely. If your friends are nice, they'll keep you in Shirley Temples or cokes all night in exchange for your services.
Keep an eye on someone else's drinking. If you have a friend who usually only has one or two drinks a night, match your ordering habits to his. Modeling someone else's behavior can help you keep track of your own habits too.