From a former cast member.
  1. Do your research, know the parks. Know what's where and when. There are double ups, you can skip something in one one and catch it in another (e.g. Belle is at Magic Kingdom and Epcot).
  2. One word: Fastpass. It's an important word. Learn everything you can about Fastpass and take advantage.
  3. Ask for a cup of water instead of a bottle. Bottles cost like $2.50, a cup is free and it's on ice!
  4. Ask cast members. They are literally not allow to say "I don't know" so you will get an answer even if they have to ask their manager.
  5. If you just want to see characters rather than meet them. Catch a parade. In fact catch a parade anyway, they're magical.
  6. For Magic Kingdom, at the end of they day, sit down watch the Electrical parade and then stay sitting for Wishes. Take the opportunity to sit down and get a good seat for the best bit of your day.
  7. If you are staying in a Disney resort any merchandise you buy can be shipped straight to your hotel. If you're not, it can be sent to the front of the park, both will avoid you carrying things around and skipping the madness of merchandise stores that the end of the night.