Often, I find complete Genesis games at yard sales & thrifts, but the cases are cruddy. When I first starting picking them up this way, I would wipe the cases with a paper towel doused in water/alcohol, but even this doesn't do the trick. Now, I dunk them in a bucket or soap and water and let them dry. (Of course I remove the insert and manuals first...)
What other methods are there out there?