Who do you tip? When? How much?

"These are the questions that have nagged at consumers since the first service transaction. The practice of tipping is meant as a form of thank-you for services rendered, or beforehand as a subtle bribe for special treatment.

Tipping need not be considered mandatory or automatic. Too often, tips are taken for granted or expected regardless of the quality of service. Tipping should be done at your discretion and as a reward for good or superlative service.

If there is one standard rule in tipping it is this: If someone renders special service to you along the way, show your appreciation with a tip."

- American Society of Travel Agents, Alexandria, Va.

Here are a couple websites with some general tipping guidelines:
Tipping Tradition – Visit Europe.com
Tips on Tipping – Travel Sense.com