This may have been discussed before, but my friend and I have used this effectively on many occasions.

If you're sitting at a bar that is full, people are always coming up to the bar to get drinks, and they typically go to the areas where there is a sufficient gap between two seated people. Creating such a gap between yourself and the friend next to you means girls are always coming to that area. All I ever have to do is give them a "hey how you doing?" and it almost always leads to a conversation of at least a few minutes. At that time you can decide if you wish to pursue it further by going for the number, etc. The only drawback is determining which of you will be talking to which girls, but fortunately I was hanging out with a married guy so he was just there for the fun .

Awesome tip. Thanks Halo!

Another one I love is "going to the bathroom frequently". You get a chance to survey the whole bar and also walk by the women's restroom where often there are women lined up who are bored and would love someone to talk to. And every once in a while you'll encounter the girl who just has to go in the men's room and do her thing. Helping her out (not literally) is often a way to make a new friend.