I'm going to be straight with you, the odds are stacked against you at a bar. Even if you're a dapper dude with a job and his own place (bare minimum requirements, in my opinion), when approaching a woman at a bar, you're automatically labeled a creep. It's not fair, but it's an assumption that unfortunately comes with the territory.
With this in mind, make your best effort to delineate yourself from the rest of the so-called creeps at the bar. If you're tearing a hole in her jeans with your Cyclops-strength grilling, you've already lost. Contrary to what you may believe, we don't enjoy being appraised as if we're a used car. The idea here is to send a short, appreciative glance in her direction and smile while making non-ogling eye contact. It should say, "I notice you" not "I've just memorized every inch of your body."