Yes, yes, yes... It's so easy to drop a lot of money on just a couple of beers in this city...
During the week, happy hour specials are your friend, so plan your night around what's cheapest. The Post's Going Out Guide just did a list of the best happy hours for interns.
On the weekends, it's difficult to find specials. My advice: Stick with light beer. It's usually the cheapest thing on the menu, even at nice places.
Oh, and don't get sloshed. It's always good to remember the next morning how much you charged to your credit card!