Life is too short to wait around for your drink. Here are a few quick tips on how to get your bartender's attention and get served quickly.
Stand near the bartender's station, where he's pouring drinks and can make eye contact. Or watch to see if he (or she) gravitates towards a specific spot when not mixing. If you're not in front of them, they can't serve you.
Having your money or credit card out shows you mean business. Waving a $20 bill at the bartender makes you look like a dick. And you'd better know what you want when asked, "What'll it be?"
Not only does this convey that you'll be back, but it means the bartender can serve you as quickly and efficiently as asking, "Another round?" And speaking of which, order a round for your friends.