Remember a little goes a long way. Circe , always a delightful source of information, supplied these hints. For scummy shower doors, use lemon oil. For ovens, take a bowl of ammonia in hot water and leave it in the oven overnight to soften spots. Or place soaked paper towels over them.
Elbow grease is necessary, and in a pinch use a sharp knife or razor to scape the worst ones off.
For walls try cleaning with only ammonia and water. Use straight ammonia on the really bad spots with a scrubby sponge. Rubbing alcohol works on a lot of the kids' wall marks like pencils, etc. Another quickie shine product is future floor polish for painted furniture, doors and mouldings, kitchen cabinets, table and chair legs.