Cleaning game contacts is easy, system contacts on the on other hand. You can buy a cleaner from ebay but it runs about $10 for a bottle. Search around the web until you find something that sounds good to you. I do not condone the use of sand paper. Good chance it will screw the contacts up. Some people used a cloth wrapped tightly over a credit card, again not a good idea. I read somewhere that someone used a thick art paper, I think it was on thecoverproject.net.
You could change the contact carriage if your soldering skills are quite good.
Adjusting them to increase the grip is quite tricky. Usually it involves a small precision screwdriver, and applying a minor amount of downward pressure at the top to the contact arch. However too much pressure and you risk screwing it up, so it has to be done slowly and methodically.
As far as carts go I use Isopropyl Alcohol in a pinch. Windex works good to, but I look at it like this.
50% Isopropyl Is 50/50 purified water/alcohol
Windex is: Water, Isopropyl Alcohol, Glycol Ether, Artificial Fragrance/Color, god knows what else. I can always cut the alcohol i use with more water if I believe it to be too harsh, and skip the rest of the stuff in Windex.
Normally I just dis-assemble the cart and use a jewelers cloth meant for removing tarnish from rare fine metals. They work great, buffs them to a shine while the cloth has a chemical in it to retard future corrosion. They tend to be on the pricey side though.
