10 - 15% is standard. Anything over that is greatly appreciated too. If you have a cook you'll need to tip him/her too.
I've guided for a lot of folks, some tip some don't, even after I busted my tail for them.
Jamiebuck makes a good point of taking them something that is not normally found in their area. I've had hunters bring me smoked trout, knives, shirts, cigars etc. Not a bad idea to start the hunt off with, that way he knows he's working for a thoughtful person.