Schultz's Encyclopedia says that a cash tip of $15-$20 per person per day is customary for an "average" trip. This seems a little low to me if the guide charges $300-$350 a day for two people, particularly if it's just a one-day trip.
Schultz's also says that giving merchandise such as fishing equipment or a T-shirt as part of the tip is appreciated in some distant locations or poor countries, respectively.

[This message has been edited by AkBill (edited 02-03-2000).]