My wife has a high gloss finish Yamaha piano, and has followed these instructions for many years with great success:
"The best approach is the least aggressive approach."When cleaning, she first uses a feather duster to remove surface dust (since the dry dust is abrasive). A soft, clean cotton cloth is her cloth of choice, and she always uses straight strokes, not circular motions. For the "dish water solution" she uses a very light solution of of Ivory Liquid dishwashing liquid detergent and water. Applying the Yamaha Extra High Gloss Polish is something she does only once per year or so (this polish leaves very little to no residue).
High Polish Polyester Finishes:
Everyday maintenance - lightly dampen cloth with water, carefully wipe piano and follow with dry cloth.
Greasy Fingers & Dirt - lightly dampen cloth with dish water solution, carefully wipe piano and follow with dry cloth.
Occasionally - use Yamahas Extra High Gloss Polish, do not spray near strings.