The advice given here is good, but is US-specific. For those with this predicament in the UK, if you're in a bar then the correct procedure is to say "And one for yourself" - this allows the bartender to take between a nominal tip and the cost of the drink. Tipping is less common in British bars than USian ones, but not unknown. Restaurant tipping follows the same rules as the US.

A complete guide to English pub etiquette can be found at http://real-ale.dreamwidth.org/1252.html