I can't really help you there. It is more a personality thing than looks. You got to look like you fit in. Nobody wants to meet the new guy, undercover cop or gay basher fears might hinder them. If the bar has a bulletin board, check it out. Even if you are not really interested in going that route, pretend to read and even jot down a number. You want to give the appearance of being on the prowl without looking desperate.
In most gay bars I have been in, you can usually strike up a conversation with someone pretty easily. Don't start off with "How big is your dick?" or "I really want to get my anal cherry busted!" Just a normal conversation about the music or the decor. After your target feels more at ease, a hand on the shoulder or covering his hand with yours for a second can signal intimacy wanted.
Before you go, make sure you have a suitable place to take your date if you get successful. Nothing more frustrating than get the juices flowing, and then standing outside the bar arguing "your place" or "mine". Don't assume that you can just do it in the dark corner, most bars have to police such action to keep from getting closed down. Cops also are aware of the gay bars and are on the lookout for male couples coming out of the bar to head into the bushes or back alley. Don't assume he will have a place without a roommate or parents. Having a place selected beforehand will also allow you to set up the room with towels, lube, dvd, or anything else to help the mood. If you get him back to your place and the action gets hot and heavy, do you really want to stop to go looking for towels, lube, condoms, etc.
If nothing else works after a time or two, drop your pants and hop up on the pool table yelling "Cherry for the taking". No, really, don't do that no matter how desperate you get. Save that for the after hours orgy after you become accepted by the bar regulars.
