The old manuals used to say to clean the contacts by passing a crisp new dollar bill through the contacts while gently pressing them closed.

Using a file can leave grooves that give a smaller contact area.

Service guys have a special file actually made for cleaning relays and contacts.

The cleaning tool is called a Burnisher :

http://www.specialized.net/specialized/Jonard-45-x-375-Burnishing-Pen-with-Six-Replaceable-Blades-534.aspx

Depends on the type of switch.

A gold flashed switch on a solid state machine, do NOT file at all, you'll cut right through the gold and then you're done. You'll be cleaning it constantly after that. Cardboard damp with alcohol is as abrasive as you want.

tungsten high current leafs, such as EOS switches or most of the higher voltage stuff in EM machines is fine to burnish.

-Hans

I've read to not file gold contact switch's. I think they are cheep enough to just replace when they burn up...

This is not subject to opinion, Mojo. Tungsten switches need to be filed. Plated switch contatcts do NOT file.
I need to find that dissertation I wrote a month or so ago Basically small surface are contatcts only carry minute currents at low voltage for signalling purposes. Clean with matte finish paper like a business card.

Large contacts like end of strokes, pop bumper and flipper cabinet switches are made of tungsten and are designed to carry large currents at relatively higher voltages and will pit from sparking and arcing. These need to be filed to a dome or crown so that they wipe each other when pressed together. This keeps them cleaner longer.

I was going to say that I used to use matchbook covers when they were available.