I just did it. All my friends were being lame tonight, and it was the only alternative.
What I usually do is, I go out and pretend like I was supposed to meet up with friends who were too drunk to find me. It is a pretty understandable excuse.
Normally, I just end up seeing people I know or recognize and I talk to them. I've also noticed girls will approach you a lot more frequently when you're alone .
The best thing to do is to not sit at the bar or stay put in one position. You'll just look like a creeper who is trolling for ass. Keep moving around, go to the bathroom line and chat girls up if you can. Switch bars if you have to. If you know a lot of people in your city, I'm sure you will see someone you know.