Leave the DVDs in their cases. Not only does this make DVDs easier to find when you want them, but the cases protect the discs from scratches and other possible damage that would render them unplayable in your DVD player.
Decide on the system of organization that will make it easiest for you to find a DVD when you want to watch it. Alphabetizing is one way to store movies that makes them easy to navigate, but you may prefer to organize your collection by actor, by genre or in order of your favorite movies.
Organize your DVDs into piles using your chosen system of organization before trying to organize them in the cabinets. If organizing alphabetically, for instance, put all DVDs that start with "A" in a pile, all those that start with "B" in a pile and continue to the end of the alphabet. This is a good time to take out any DVDs that you no longer want so that you don't have to rearrange the DVDs again later.
Place the DVDs into the cabinets. While you can lay DVDs any way you like, keeping them upright in the cabinet and putting them side by side makes it easy to slide DVDs out when you want to watch them and return them to position when you are done.