It may be a bit time consuming, but a little isopro and/or distilled water with cotton swabs is a safe way to go. (Personally, I prefer not to submerge PCBs in bathtubs or dishwashers.) The liquid should evaporate quickly without leaving any kind of film.
I'll usually stick with just the distilled water anywhere there's anything printed (tops of chips, etc) just to be sure the IPA doesn't take the print off.
Afterwards, a little compressed air now and again will keep it looking new indefinitely.
-edit- One more thing to add, if you're dealing with food stains, distilled white vinegar annihilates them pretty quickly. (I usually use a mixture of distilled water/white vinegar.)