I'll confirm Chuck's comments about abrasion/mechanical cleaning, and problems with overheated contacts that have annealed the spring action.
The primary product I use is Caig Deoxit D5.
http://store.caig.com/s.nl/sc.2/category.188/.f
Caig products are well-known in the electronics industry, and recommended by computer and instrumentation manufacturers. Use sparingly, and one tin of D5-S6 will serve you for years.
Many connectors can be disassembled to get access to the metal parts. Look horizontally into the connector with a bright light, and if you see a spring tab holding the metal piece in the connector, use a jeweler's screwdriver or a paper clip to depress the tab and slide the contact out of the connector body. Make sure you reinsert the contact in the correct hole in the body after cleaning, and that the tab locks the contact in place.
Files, sandpaper, pencil erasers, and such are not advisable. If I need to remove corrosion verdegris from a brass/bronze connector, I use a toothbrush dipped in household ammonia. Rinse with water, allow to dry, and finish with a shot of Deoxit on assembly.
About the only solution for badly-overheated and annealed connectors is replacement.
Hank