Milking is when you frequently use one brand recognition to sell other genre of games and forms of entertainment. Typically Mario as a character is being milked because Nintendo knows that everybody loves Mario so they adapt it to any purpose (Mario & Sonic, Mario Kart, Mario here and Mario there). Halo wasn't being milked untill MS had the idea to "expand" the Halo world and make it its cash cow, selling it in the form of a RTS, a remote spin-off like Halo ODST, implement it in the XBL, create the animated series, etc. Guitar Hero is typically milked since you have 365 installements to say the least plus Band Hero, DJ Hero and the soon to come March Band Hero or whatever. Final Fantasy is in the very same boat.
But Rachet & Clank is not a milked series, all games are plateformers, all are tied together in a big serialized adventure and the brand is not used to cash in on other products.
