1) Have a drink that you can order with confidence. Make sure this is a cocktail that is relatively easy to make, and classic in style. You also want to order something that makes you look professional. Stay away from anything that uses coke or fruit juice as a mixer. My go-to cocktail is a Manhattan. If you aren't into something that big and bold, you might consider something in the realm of vodka tonic, Campari & soda, or simply your favorite (which should now be Ketel or something similar) vodka on the rocks. Under no circumstances should you order Rum and Coke, Vodka and Orange Juice, or a Singapore Sling. You are out of college- leave those drinks behind. Also, no shots are to be ordered. Ever again.
2) Only have one or two of your cocktail. After this point, things may start spiraling out of control for you. If people are pressing on, switch to beer.
3) If you order a beer, make sure it's decent. Again, leave the Bud Light in your college apartment, or your fridge at home. If they don't have a craft beer at the bar (they should), then at least order something that is locally made, or Guinness.
4) Don't get drunk. This is a touchy one, as the people who invited you might have too much to drink. However, if you cross the line, they will see you as less-than-professional, and won't forget. One easy way to keep this in check is to drink a full glass of water for each cocktail or beer you have. This will slow your consumption rate.
5) Be wary of wines by the glass. This totally depends on where you are. Most restaurants basically don't have very good wines by the glass and are making a ton of money on cheap product. If you at a wine bar, or a place that specializes in wine, throw this rule out. Also, don't ever, ever, ever order "House Red" or "House White". That's Busch league.
6) Offer to buy a round. When you do this, don't say anything about the prices of the drinks- they will cost significantly more than they do at your corner bar. Everybody already knows this, by mentioning it you will look like a rookie.
7) Have something to talk about. There will be some work talk, but also have a few subjects that you have brushed up on. One good way to do this is to read the front page of MSN.com, and ESPN.com so you can know what's going on in the world and in sports. Just like Momma told you- stay away from politics and religion. These usually just get people riled up, and aren't appropriate for most business situations. Oh, and don't talk too much.
If you follow these rules, and actually smile and have a good time, you will be just fine.