You know, the ones, where all you can see is their back?

From the sides or somewhere they can see you. NEVER EVER from the back.

bump. thanks Johnny, but sides is often not an option if bar is crowded.

anyone?

Johny is right. Squeeze in next to them to order a drink, either comment on their drink selection or their general mannerisms. I hope you're quick on your feet cause you probably will face a shit test.

thanks. But theres gotta be players out there who know how to approach when sideways not an option!

Make them turn round, have a conversation behind them with a wingman so they overhear. One thing that always makes them turn around is when they hear you saying that such and such woman is easiest the best looking most attractive woman in the bar.They will turn round to see who you're talking about (it's a hard wired rivalry thing for women) You've now also created an opener for discussion.
If they are HB9+ make sure the woman you are talking about is an opposite type to them, IE if they are blonde pick a brunette, if they are tall pick someone petite. You have now stealthed a disqualifier.

If you can't go from the side, you've essentially gotta do a blind approach. And possibly eject quickly. Just walk up, slightly touch two of them on their shoulders, lean in between them, and start a conversation. Since you're already being pretty straight, might as well go direct.