I usually use a cloth wrapped around a credit card sprayed with Deoxit D5 and insert and remove until clean. If I can't find the credit card I use sturdy cardboard sprayed with the contact cleaner.
With my one slot I just take the board apart and clean with a medium bristle tooth brush and high concentrate tubbing alcohol. There are a few posts of here and on Neo-Geo.com
You only need friction if the slots are corroded or otherwise in horrible shape.
I clean the slots with a toothbrush and some liquid dishwashing soap. Rinse well then dry in the oven (without the battery of course) at 170 degrees
If you have an electric oven then either put the board all the way at the top rack slot OR put a cookie sheet between it and the heating element as the radiant heat from the element may be too much.