How much to tip?
In the United States tipping of many service people, including waitstaff and bartenders, is part of the social contract we enter into when we decide to patronize a full-service bar or restaurant.
In U.S. restaurants where table service is provided, 15-20% is the standard for good service; 25% for outstanding service.
This also holds true if you eat at the bar, particularly a bar within a restaurant or hotel. The bartender who takes and places your order, then brings out the food, is doing no less than a waitperson who takes care of tables.
If you're only drinking at the bar, the same percentages should apply with an exception: if you're knocking back $4 "happy hour" specials, $1 a drink should be the minimum tip.
While tipping is always at the guest's discretion, if you received substandard or poor service, don't just "stiff" your server: speak to the restaurant manager. Otherwise, the server will never know why they didn't get a tip, and the restaurant personnel will be deprived of information that could help them improve their service.
There are also other circumstances where tips are warranted. At airline clubs, for example, where complimentary drinks are often a perk of membership, it's appropriate to tip the server who is pouring or mixing your beverage. $1 for a simple cocktail or poured glass of wine or beer will be appreciated. If the drink you've ordered involves more preparation, such as a Bloody Mary or a margarita, a $2 tip -- for the additional effort -- is better. After all, the drink itself is free.
The minimum tip guidelines outlined above also apply at a "cash bar" at a reception or banquet. However when attending a hosted reception at a hotel, restaurant, or banquet facility, tipping is not necessary (though leaving a dollar will be appreciated and usually get you remembered in a good way). The reason is that the host will be charged a percentage of alcohol sales -- usually between 15% and 20% -- and that percentage will be divided among the bartenders who worked at the event.