http://www.amazon.com/Nintendo-NES-Cartridge-Slot-Replacemen...
https://nuxx.net/wiki/NES_72-Pin_Edge_Connector_Replacement
I did it on mine in college -- and then I did it for my suite-mate after he noticed that I never had any problems with my NES -- and then I did it for everyone on my floor who had an NES... :) Works like brand new.