Book-to-Movie Adaptions: Which do you prefer to do first?

I have wondered about this all my life and I have never really found a solution. When books are made into movies, which is better? Reading the book first and then watching the film or watching the film first and then reading the book?
In most cases with me, I'll have read the book a while before the movie came out or before they even began making the movie, as it is with Vampire Academy, Hush, Hush, Fallen etc. All these books will be made into films within the next few years and since I've already read the books, all that's left to do is see the film.
But when you're being introducing to a new book/movie, what's better? I find that if I've read a great book that I really enjoyed, I have high expectations for the movie and it usually lets me down. This happened with Twilight and a few others. 
However, when you've see the movie first, it ruins the plot of the book for you.
So what do you prefer to do? 
On another note, I've never really been much of a fan of movie-to-book adaptions. Come to think of it, I don't think I've ever actually read a book that was published after the release of the movie, based on that movie. 
Again, what are your views/opinions on this?

In this post, I also thought I'd let you know my top 3 favourite book-to-movie adaptions so far, so here's that. 

1. The Hunger Games
2. I Am Number Four
3. The Perks of Being A Wallflower 

What are your favourites and least favourites?

Please, please, please let me know down in the comments what you think and as always, I reply to every comment I receive.