Unless other rules apply, the list that JohnnyRocket linked is the law. Leagues or tournaments may have additional rules limiting what you can use and when. In tournaments, you are allowed to clean your equipment by more than just wiping it with a towel as long as they have no rule prohibiting it.
You're still restricted to "approved at any time" cleaners, though. Cleaners on the "Only before or after competition" cannot be used during a game or block (and doubles/singles almost always count as a single block).
I believe Nationals does not allow you to clean your equipment during play.
SH