It's important to know when to clean your sword and what not to clean on your sword. It's also important to know what you can repair on your sword and what should be left to a professional.
When to Clean and When Not to Clean Your Sword:
When handling has caused light rust.
When moisture in the air causes light rust.
When a new sword has light rust on it.
After finding an area that is just starting to rust.
If the rust is heavily scaled.
If you are willing to accept that there is a chance that you may damage your blade and may diminish its value.
Don't try to polish any brass parts of the sword (it destroys the patina and looks really bad in conjunction with a rusty blade).
Don't try to remove the patina from the blade.
Don't try to make an old sword look new -- it isn't! The patina is often seen as an authentication of age.
When in doubt or before any heavy cleaning, consult a professional.