I use contact files - as in the METAL file - I use for high current applications such as the slingshot contacts and flipper button contacts.
Flexstone contacts are good for many applications where there a very worn contacts.

I use thin cardboard as most contacts are in excellent shape, but they have built up dirt, lube and other crap that simply require the cardboard to "dislodge" the dirt. All the contacts in the score reels were cleaned with cardboard. The relays - I used Flexstone in some cases - you can tell - where the point on the leaf contact is larger.

Cool, cheers for the info, hope this helps others too 
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Cardboard here too.. namely a strip off a business card or similar dipped in meths. Lifts the dirt and grease off easily