When cleaning my game carts for Nintendo systems, I use a 3.8 mm gamebit head to get everything apart. Then, I take out the circuit board, and clean both sides of the metal contacts with Brasso polish. After that, I use a 1/1 mix of alcohol and water with Q-tips, and dry everything with an old shirt or soft cloth. If needed, I may also clean the inside of the cartridge shell with just the water and alcohol. Once everything's done, just screw it all back together, and that's it. Hasn't failed me yet.