Set up a comfortable space for reading. This can be a favorite chair, a bed or even a coffee shop.
Start reading at a manageable pace. Don't try to zip through the story. There are so many plot threads flying through the book. Confusion and frustration can result. Try starting with one to three chapters per sitting, and see how that feels.
Take notes. This may be reminiscent of school, but it may also be what keeps everything in order. There will be no test--assuming this isn't for a class--so just jot down the important people, places and things. When you see references to them later, there will be no confusion.
Enjoy the language. Be sure to take the time to let the language of the novel flow. Books just aren't written this way anymore and because of that, the novel may seem too dense. Soak up the words, and let those words paint a picture of the times.
Celebrate the fact that you've completed War and Peace. Do something special to celebrate the achievement. While it's enjoyable, it's not always an easy task, but the sense of satisfaction is well worth the time spent reading.
Rent the movie version. There are two solid movie versions of War and Peace--the 1956 American film and a 1968 Russian adaptation.