What's a good rule of thumb for tipping tour guides (and drivers)?

On a short bus tour (several hours or less), tip your guide 10 - 20 percent of the cost of the tour. Give it to him or her when you say goodbye. Charter and sightseeing bus drivers are also tipped in certain cases: when drivers double as guides, $1 per person per day. When the driver has been particularly amiable, the person in charge of a private charter sometimes asks each passenger to contribute $1 or more to a tip pool. On a longer tour with no built-in gratuity, each passenger should give $5 - $10 to the guide and another $5 - $10 to the driver.