I care a towel that is wet on one end and use it to wipe down my clubs after each shot. If there is any build up of dirt, I soak them in warm water and scrub with a brush.

David D said:

the only time fitting is truely a must is when you can repeast your swing consistantly and not before doing swing changes and buying off the rack is perfectly fine for a beginner but if your shooting low a fit might help some inconsistant…

There was a study in Golf Digest a few years ago showing how water affects golf balls, I dont remember the results though.  I doubt anything could be done to increase resistance without sacrificing performance.