They improve the graphics, continue the storyline, possibly add new weapons, enemies, characters etc. and new vehicles. With movies, it keeps the good storyline and it survives. It was that way with Lord of the Rings and Star Wars, among other series. The same concept goes for video games. They just need to make sure they don't sacrifice quality gameplay for storyline, or storyline for gameplay. That's when it is just milking.
Edit: In case you don't get what I mean, a prime example- Pirates of the Caribbean. The first one was a pretty good movie, second was okay, third one was horrible, didn't even bother seeing the fourth. They kept all the fun action throughout the series, but they completely dropped the good storyline.