If you're interested in becoming a face character (a princess) at Walt Disney Land or World keep reading to learn how.

A job as Cinderella at DisneyLand

[edit] Steps

Attend a Disney audition. Go to the website http://www.disneyauditions.com to see a list of up coming auditions.
Look carefully because each audition is looking for something in particular (i.e. parade performers, male comedic actors, etc.) You may need to travel.
Arrive early and dress appropriately for dance and an improvised scene.
During the character audition you will learn a short dance, and also act out a "fun" little scene. The first part of the audition is the dance. Although it is not very complicated, if you are unable to master the dance you will not be able to continue to the second part of the audition. After the dance, the "judges" will give you a scene they would like you to act out (eg. Goofy getting/ decorating a Christmas tree, you getting your favorite dessert).
Study your dance and flexibility for the audition. You don't need to be a professional dancer but a background and experience in choreography helps
Be animated. Bring the characters to life.
Wait. Judging takes some time. After the audition it is pretty much a waiting game. Disney does have specific things they look for in each princess. However, overall they don't want anyone with defining features. For example, if Cinderella has a huge mole on her face one day and then it is "gone" the next day, children may get suspicious.

[edit] Tips

Be as animated as possible during the audition. Don't be afraid to look like a fool. This is the part were you show Disney that you truly can become someone else and not care what people think of you.
Many train as fur characters before moving into the face roles. If you do get chosen to be a face character don't expect to become a princess right away.
Princess gowns only go to a size 10. Which is smaller than a usual clothing size ten.
Visit the gym and get into the best shape you can.
Sign up for dances lessons. You will learn how to execute some common dance moves you may see again at the audition. Plan to start your dances lessons a year or more before you start auditioning. A few won't be enough.
Take drama classes at school or sign up for acting classes in your town. Look for classes that offer techniques in improvisation and most importantly, pantomime techniques.
Rent videos and research the characters at Disney. Look at their moves and how they interact with the guests. Keep those techniques in mind at your audition.
Talk to your drama teacher at school and ask for advice.
Have head shots taken and take them along to your audition. Don't feel pressured to spend a lot of money on these. Have a photo taken and take it to a local copy shop to have reprinted on 8"x10" glossy paper. You can have several head shots made for less than $5.
Make a resume. If you've had a lot of stage experience, dance experience or experience you'd like to share you can create a performance resume of your work. This can be printed on the back of your head shots or included on a separate sheet of paper. Format it like a resume and make sure to include your name, address, email and phone number on the page. If you are underage you can also include a parent's contact information.
Look at the fine print. If it says to bring a parent you should. They may need to sign paperwork. This isn't an opportunity for a family outing so leave friends and other family at home. Auditions are hard enough without having everyone's younger brothers and sisters running around.
Wear appropriate shoes. If there are dance auditions you should plan to wear a pair of jazz dance shoes or other footwear for dancing. Athletic shoes, dress shoes or fashionable shoes may not be appropriate for the dance moves you'll be learning.

[edit] Warnings

Beware of agencies that guarantee roles or guarantee auditions if you pay a fee or buy a photo package. Many fraud schemes take advantage of eager teens and their parents by making promises for jobs at Disney. Only trust official the official Disney site.