Smile, get eye contact
DO NOT wave money, bang the bar or be any kind of nuisance, you have no idea how fucking annoying it is having some asshole shout at you while you're busy doing something else or seeing to someone else, and in most cases most staff will make you wait longer out of principle
If you're not being served it's probably because the bar is very busy, in which case it's just better to be patient, if you keep coming back try to get the same guy or girl, they should remember you and remember the fact that you're not an asshole and be more inclined to come to you first, it's also good to leave a small tip like £1 or $1 or whatever, it's not much but this is what will get you remembered and faster service in the future
Bottom line, don't be a twat