Always a good question that comes up often. After being in the industry for over 30 years i would like to make a couple of point.
If the fishing is slow, don't blame the guide......unless the other boats are all catching fish except you. Most fish are migratory, he can't catch them if they aren't there. Base your tip on his effort. Is he checking the gear often? Is he changing up his presentations? Is he networking with other guides to see if there is a bite going on somewhere else? Is he a friendly, service orientated person that is fun to spend the day fishing with?
I usually suggest a minimum of $20 per person but more is always appreciated.
Have a great trip!