Over the years, pianos accumulate dirt and grime inside the piano case and in the moving parts of the piano. This can prematurely wear out felt, leather, and wooden action parts. In addition, insects often lay eggs inside a piano and mice love to tear felt out of piano for making their nests. Coins, paper clips, pencils and just about anything else you can imagine fall inside the piano. A full cleaning includes polishing the cabinet and the keys, and thoroughly cleaning the inside of the piano, including the piano action. This will make your piano look years younger, and make sure foreign objects are not causing potentially expensive problems.
The best way for you to clean dust and finger marks off the piano is with a soft, lintless cloth slightly dampened with water and wrung out. Follow that with a soft dry cloth to remove any remaining moisture. The keys can be cleaned in the same way. Most piano manufacturers recommend against the use of furniture polish. Cleaning the soundboard or piano action requires special tools and training and should only be done by a qualified piano technician. This should be done every 5 to 10 years.