I use the cheapest method and it does wonders
91% Isopropyl alcohol and some Q-tips also called cotton swabs, just takes some of that kind of alcohol, then pour it into a small cup. and then dip the Q-tip into a cup then move the swab back and forth on both sides of the game cartridge. When you see that your cotton swab is black on both sides just throw away. and repeat this, until no more black guck is coming off the game cartridge. Never use Cotton balls because that is more likely to stick to the cartridge.
I used this technique on all of my old NES cartridges I purchased from different places, where they didn't clean anything before selling the games, I only cleaned them once and never had to clean them again because I've had them for about 5 years and they still are pretty clean.
Also you can buy Cartridge tip covers, but I don't know where to buy them reasonable, I haven't looked but I got one cover from A SNES game. I bought a long time ago also you can get NES ones but I never seen a N64 cover before