It seems to me that whatever house your parents were in is where you will end up. I know the Sorting Hat initially wanted to Sort Harry into Slytherin, but that had a lot to do with his connection to Voldemort. He ultimately ended up in the same house as his parents (as all children seem to), Gryffindor.
The Weasleys (Ron and Ginny), and later the offspring of the main characters are all bent out of shape over the Sorting. Ginny is congratulated by her mother for being Sorted to Gryffindor, despite the fact that Ginny already had six brothers in that house (would she have been chastized for ending up in Hufflepuff?). What were the odds she would have ended up somewhere else?
When it comes to Sorting students into their houses, the students seem very concerned about what house they will wind up in. What are these kids so worried about?