I have not technically clicker trained any of them, but I have taught them to respond when I snap my fingers. I mainly started it as a redirection technique for a stalker cat. The best tip I can give is to be quick with the positive reinforcement so that they learn faster.
I don't but the woman who runs the local TNR group trains all ferals who need to be trapped to come to a clicker to be fed. It also allows all colony cats to eat at a certain time and to remove dishes which might be an issue for local people. She swears by it.