There are a lot of myths out there about cleaning cartridges and making games work better…
- Blowing on them does nothing
- Q-Tips and Alcohol will not fix them
- Inserting/removing multiple times is not a true fix
- Contact cleaner does more harm that good
All blowing does is remove loose dust. It does nothing to remove dirt, gunk, skin oils, or oxidation.
Q-Tips and alcohol will only remove some oils and gunk/dirt that isn’t attached well. It doesn’t do much. Here’s a picture of a dirty Neo-Geo cartridge edge connector:
Here’s what came off using a Q-Tip and 100% Isopropyl Alcohol:
Here’s what came off AFTER using the Q-Tip and Alcohol when I used Goof Off:
These contacts are designed to be self wiping/self cleaning on insertion and removal and #3 actually will work on most systems but is only a short term fix. It doesn’t work at all on a NES game console unless you have a top loader. The front loader doesn’t wipe the cartridge contacts on insertion/removal. When you push the cartridge loader down on it the contacts get pushed onto the cartridge and don’t wipe across.
The proper way is to clean with a pink pencil eraser. It’s very lightly abrasive, will not remove metal, will remove embedded gunk/dirt, and leave everything nice and clean:
I NEVER recommend any sprays such as contact cleaners and NEVER any oils. They will just attract dirt faster and cause more problems. I’ve seen some severe board damage where a spray was used and exposed copper traces were eaten. That board was unrepairable. 
If you have corrosion on the edge contacts you can use a fiberglass pencil that you can pick up at Fry’s. It works VERY well at being more aggressive and is far safer than sandpaper.
Now, we can’t always take apart the cartridges for fear of destroying value by putting holes in labels to remove screws. In those cases I use Goof Off and a Q-Tip, but take care as it can damage/dissolve plastic cartridge housings.