Seriously, don't put anything oily/waxy on the fingeroard, or anything that can build up and clog the strings. If you want to polish the rest of the instrument (not the head!), bear in mind that the only way to get under all that metalwork is to dismantle it -- don't, unless you're planning a major overhaul. Avoid ending up with polished areas and mucky areas that you can't quite get in to (i.e. don't polish it!).

If the headis plastic, a damp cloth ought to clean up coffe stains and the like; don't use any kind of cleaning fluid, unless it says it's suitable for musical instruments (unlikely). If the head is skin ... I'm not sure what to use on it. Sooner or later, a banjo cleaning expert is going to post to this thread. (I'm just good at getting them dirty!)

Steve