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!