i used hot hot water with a tooth brush until i found a cleaner designed for removing resin.
I would take the bong apart soak the smaller pieces in hot water while i ran the bong itself through hot water while i scrubbed the inside with hot water. rinse and repeat as required. then i would take the smaller pieces out from soaking and scrub them the same way. just remember to keep the water running it helps to keep the resin from sticking to the sink. fyi use gloves if you can. keeps the smell off your fingers.