Use a special cleaning solution, such as Key Brite from Cory Industrial Products , usually available from your tech or a piano dealer. Use a slightly damp cloth and gentle soap. Clean each keyslip individually and wipe it dry immediately. Use a different cloth for black keys, in case there is some dye which is transferred. The casework should be cared for as you would a piece of fine furniture.
Some manufacturers sell cleaning products specially made for their finishes.
Call your dealer or manufacturer or check with your piano technician. Also see Cory Products. Many piano manufacturers advise against a regular furniture polish, but I have had no problems with a cream-type product. Obviously, a spray-type product will be impossible to control. Follow the same techniques, but with extra gentleness and care.