1. It can't come from way out in left field: Whatever your surprise is, it still has to make sense with the story. You don't want a soap opera where you keep killing off characters, but they also keep coming back to life. This sort of thing needs context. But then, you also don't want a gangster story to suddenly be invaded by scifi and have a character abducted by aliens. (Although, this would be a fun contest idea. Combine two genres that don't usually go together. Hmmm... putting that away in my own personal suggestion box.) Make the surprise make sense for the context of the story.

2. You can't have so many surprises that the storyline completely haults. The storyline has to keep on trucking. As they say in lots of movies, the show must go on! The storyline must keep moving despite the surprises, although it isn't a bad thing to instill a fear into the reader that the story can't go on with the surprise happening - just don't forget to have a character work it out.

3. Don't forget to resolve whatever it is. If you have a lot of surprising happenings, that opens up questions and additional side storylines. Don't forget to close those up properly. You don't have to do it right away, it can take some time, but all of those loose ends need to be tied so the reader feels satisfied when they come to the end of the story.