There is a big difference in forgetting to use an ability of the character class and forgetting how the ability works. Both are the character's issue. I'm not going to remind a player that he forgot to rage his barbarian. Nor am I going to remind a character how to do a Trip attack. That's his responsibility. If he doesn't know how to use the maneuver, he shouldn't be using the nmaneuver.
In the game I play in, there is plenty of time between my previous turn and my next turn for me to pull out my PHB and check up on what I want to do. Granted, it invariably comes down to my turn and I can't do what I wanted because of some action of someone else.
If your guy forgets how to do a trip every time, have him draw up a cheat sheet, check it over and whenever he wants to do a trip, he can pull out the sheet. I did one for grappling.