I use a product called Minwax Wipe-On Poly which seals and protects (pretty much for life) both stained and natural wood. It leaves a rich finish, enhancing + preserving the natural grain and texture of the wood. More coats make the finish more glossy, fewer coats (1-2) leave it matte / more natural, which is classier in my opinion.
I would only recommend it if the current finish is pristine (or if happy with it) because it will preserve it. I like it because it also adds durability and protects against moisture damage etc, but the permanence may not be for everyone.
If you're after a more mutable solution, something like what rottalpha suggested would be better.