This may come as a shock to some, but it bears repeating over and over again: most bartenders aren’t getting lavish hourly wages. If you’re getting good service, tip your bartender -- and I’m not talking about a quarter here and a quarter there. Give them no less than 15 percent; 20 if you’re classy.
Don’t be indecisive:
Know what you want when it's your turn. When the bar is packed and bartenders are in a rhythm, few things aggravate them more than the patron who's had plenty of time to decide what they want, but still approaches the situation like it's the first stop on their 21st birthday pub crawl.
Don’t ask for a “strong” drink unless you’re prepared to pay for it:
"I'll take a vodka and tonic strong."
You’re essentially doing two things when you say that:
1) You’re asking for something extra for free.
2) You’re suggesting that any previous drinks made by said bartender were substandard.
You wouldn’t go into Subway and say, “I’ll take a cheesesteak with extra meat,” and not expect an upcharge. Instead of asking for a “strong” mix, just order a double.
Don’t assume all bartenders know every drink ever made:
For one thing, some cocktails are known by multiple names. Then there’s that wacky concoction that one guy made for you when you went to Put-In-Bay three years ago ("I think it was called the Purple SomethingOrOther, and it had vodka- no, gin and stuff in it.")
Some bartenders are great at improvising and trying to get a cocktail as close to what you had as they can. Others are just as likely to throw vodka and some sort of juice in a glass and call it a day.