The best way to keep your futon plump, comfy and stain-free is to cover it and flip it regularly. Purchase a futon cover if using your futon as a couch, to protect it from stains, dust and oils.
Most covers are a little thicker than a fitted sheet and are machine washable.
Buy a mattress pad or a vinyl sheet to protect it from stains if the futon is being used as a bed. The sheet or pad goes between the fitted sheet and futon mattress. Flip your futon once a month if you are using it as a sofa. Rotate it each time you change the sheets if the futon is being used as a bed. Hang your futon outside in the sun to keep it fresh.