A small amount of Windex OR Isoprophal Alcohol(you want the 91 or 97 % lower has to much water or other liquids mixed in and to high can damage contacts) on a q-tip or on a thing called a one up card(it comes with the alcohol if you buy them)will USUALLY clean them well enough to work. If you use Windex make sure it is BRAND NAME WINDEX or WINDOWLEAN in Canada. cheaper Glass cleaners will have to much water and rust your pins. If they are extremely dirty you will have to Open the cart up and then using a Brass Polish is best though in a pinch you can use a Pencil eraser though I wouldn't suggest using that every time on the Pins being very careful to only get the polish on the Brass Pins.