If you want to remember everything about your prom night, it's a good idea to start thinking about it early so you can begin saving keepsakes. One of the best ways to remember your whole prom night experience is to keep a journal. You should begin writing your journal months before the actual prom so you will have plenty to refer back to later on. Record all sorts of details and thoughts (even the little things). Later when you look back on this stuff you'll be glad you wrote everything down. Your journal can then be supplemented by a scrapbook with photos, flowers, and other mementos.
For your journal you may want to buy a separate book to write in. You are going to be surprised at all the things that happen to you in the next few months. A journal is a nice way to keep in touch with your emotions.