Go to the Disneyland admissions web page. Note the cost of an annual passport, which as of September 2009 costs $429 but allows admission on any day of the year.
Choose the passport that meets your needs, but do not buy it yet. Cheaper versions restrict your entry to specific days. If you are a resident of Southern California, you can buy the cheapest annual passport at $144, which allows entry for 175 pre-selected days.
Sign up for the Disneyland Birthday Club using the link under Resources. This allows free admission only on your birthday to either Disneyland or California Adventure, but not to both. If you are 13 or older, you can sign up by yourself. If you are 18 or older, you can sign up your children.
Wait for the free admission confirmation to arrive about two weeks before your birthday. Print and save it.
Go to the Disneyland ticket booth on your birthday with your voucher and a government-issued photo ID that shows your birth date, such as a drivers license or passport. Show both to receive your free ticket, which is worth $72 for adults. If you somehow forgot your voucher, simply tell the ticket booth that you've already signed up and show your ID.
Upgrade your ticket to any annual pass. The $72 cost of your free ticket is applied to the cost of the pass. As with any other upgrade, you have until the close of the park to upgrade but avoid doing so at the last minute to avoid the rush.
