Most bartenders are living off their tips, so don't be a cheapskate you dicks. Also I've made friends with most of the service people at places I frequent so I end up over-tipping a lot of the time. If I'm in a place where I don't know them I tip $1 a drink or sometimes $2 if it has red bull. Anything more than 2 drinks and I try to shoot for 20%. If you are paying cash you will tip more this way, so it's better to just run a tab and figure the 20%.