1) Wear comfortable shoes. Most people wear athletic shoes. You will be on your feet for hours.
2) Pack in a water bottle. Drinks can be very expensive in the park. It has been our experience that security won't ask you to throw out a water bottle.
3) Plan ahead! Disney's website (http://www.disneyland.disney.go.com/) is a great tool. You can find maps that include restaurants and shops, along with all of the rides. It will also tell you which rides are the most appropriate for children, teens, etc.
4) Budget enough money to buy food in the park. Yes, it is expensive, but we have found that it is worth it! It will save you from having to leave the park to eat and everyone is happier when they are not hungry.
5) Use the Fast Pass System. Really, it is the only way to go!
6) The parade is worth watching...once. If you are going to be at the park for more than one night, ride the rides during the parade. Lines are short and you can usually walk right on!
Enjoy. Enjoy. Enjoy. Disneyland is truly the 'Happiest Place on Earth'!