A can of compressed air is the only real way to clean components/memory on videocards or just about any electronic component. You don't want to blow with your mouth though as you'll get saliva that could short fragile components that way.
Pick-up a can of compressed air if your memory chips, gpu fan or heatsink are getting clogged up with dust and debris.
If you're talking about cleaning thermal compound- as in you had heatsinks on your memories and installing new ones, with the board outside the PC- use isopropyl alcohol with a lint-free cloth to clean off any thermal compound. Let this dry fully before applying new compound if affixing heatsinks or memory cooling.