Reddit mentions of NES Cleaning Kit by 1UPcard - Console and Game Cartridge Cleaner Bundle

I mean, I basically outlined it in a previous post here. Take a clean cloth and some isopropyl and work it in to the connector.

Some people use an old credit card or something like it and wrap the cloth around it and work it down in to the connector where games get inserted. You could also use an old toothbrush instead of a card and cloth. You could also use something like this.

Then take a small regular screwdriver (like for eyeglasses) and gently lift the pins uniformly across the whole connector. Also clean the contacts where the lower pins sit on the motherboard with isopropyl and a few q-tips to make sure you lift any other nasty bits off the board.

The single best thing you can do once you get the connector cleaned is to clean ALL of your games with isopropyl and q-tips. Try not to put dirty games in clean connectors.