Do NOT use brass polish!! It and the pencil eraser will leave a film on the contacts! And abrasive paper is also not a very wise idea, it will also cause contact wear!!NO NO NO!!
For contacts that are realy bad, use a Fiberglass pencil (sold in electronic stores).
Do not rub the contact too hard with this pencil, just let the fiberglass do the work!
To clean the surface use compressed air to remove any leftover fiberglass from the board.
Follow up with a Q-Tip with Denatured Alchol (not rubbing alchol it will also leave a film).
Also when working with any eletronic boards, make sure you are properly grounded with a wrist strap to prevent static dis-charge from destroying the board.
The above is especialy true in the winter months when the humidity is low.
Hope this helps!