Position it away from any radiators.

Keep the lid closed to stop dust getting in the works. If the keys are ivory, they naturally yellow over time; newer keys will be plastic.

Either way, wipe clean with a just-damp chamois leather.

Remove any dust from inside the casing with a vacuum, or blow out with a hairdryer set at cool.

Finally, the best way to keep a piano in nice condition is to play it!