This assumes you can get the guy to look at you. Half the time they are very busy and just keep walking past you. In that situation I suggest starting a small controlled fire.
I'd love to see this guy at last call ... most probably standing there dry.
Order the same thing all the time. If the bartender knows your order, he'll pick you out of a crowd of people waving cash and make yours first. Under less urgent circumstances, he'll get your order with just a nod.