2.) Dip one of the fuzzy cotton ends of a Q-Tip into the rubbing alcohol.

3.) Slowly rub the wet end of Q-Tip on the contacts of the game cartridge. Slide it back and forth touching every receptor.

4.) Flip the Q-Tip over to the dry end then soak up the rubbing alcohol on the contacts of the cartridge.

5.) Keep repeating the first four steps until the Q-Tip comes out clean.
Cleaning Cartridges If the NES still doesn't work, the contacts inside the machine may be dirty.

2.) Remove the lid so you can see the "guts" of your Nintendo.

3.) Remove the metal shield protecting the innards.

4.) You will now be able to see the black part where the cartridge slides into the machine. At the back of it you will see the gold colored receivers for the cartridge. Get a soft bristled toothbrush, that you don't care about, and dip it in rubbing alcohol.

5.) Slowly poke the bristles into the slots where the cartridge is inserted and carefully scrub.

6.) Reassemble your NES and let the contacts dry off before you use the system

I've tried the above methods and my NES still doesn't work. Help!
It could be that the NES system's 72 pin connector is shot. In that case, do a search on eBay or securely order online @ MCM Electronics, part # 83-3785, for a new connector. If you're feeling lucky you could try getting in touch with Nintendo's hotline first (contact found on Nintendo.com) and be real nice and they might just send you one for free.