I take sequels on a movie for movie basis.
I think a squel has to build upon but be as strong or stronger than the first film in terms of plot, characters, action etc. They have to be good stand alone films.
It can be hard to do and can fall down, examples being the Star Wars prequels, Indiana Jones 4 and the third Jurrasic Park film.
I await Transformers 3 and the next Star Trek film to see if they can keep a good run going.