Bartenders
Respecting the industry average of $2 to $3 a drink is pretty key. Going above, however - say $5 or $6 a drink - will not go unnoticed.
Generosity with bartenders could quickly equal shorter waits in line, and who knows, maybe even score you some small talk that would lead to a first name basis sort of relationship.
Because really, don't you just want to go where everybody knows your name?
[Photo via @theodeonnyc]