You shouldn't ASK for a comp at all. When it's time to leave, you take your credit card out and wave it towards the bartender. The bartender then waves you off and thanks you for coming in. You then leave the bartender a tip.
That's just bar etiquette, owner.
And if you do all of that and the bar doesn't comp, then don't bother coming back. It's very simple.
Or you can walk out without paying your bill, have a discussion with the cops, or even worse, have everyone in the bar go the media and the internet and talk about what an a$$hole you are.