1. Choose a good 3 hour movie, like Ben-Hur, Barry Lyndon, Lawrence of Arabia, any Godfather movie, an extended version of Lord of the Rings, or Gone With The Wind. It needs to be something you're interested in. Read a summary of the movie beforehand so you're sure that you're willing to sit through it.
2. Put your movie in your DVD player. Make sure its free of scratches, or dust. Skip the previews, they only add to the length of the movie.
3. Have some snacks handy. Any kind of popcorn, candy, or even some veggies are great movie food. Have a glass of water and some napkins handy. Dinner is also a great pair for long movies.
4. Take bathroom breaks when necessary. Take one before you start the movie, so you won't have to go all the time. They will also give you a moment to stretch your legs and relax your mind.
5. Don't skip through chapters. Skipping chapters can result in a disjointed plot line. If the movie really doesn't interest you, stop watching.
6. Pause the movie if you want to quit. Try taking a small break, stepping outside and stretching. Even in you aren't interested in finishing at the moment, you may want to return to it later.