Catholic Church considers you Catholic for life if you were baptized UNLESS... from Roxanne DeMonica's blog

It involves a little bit of paperwork and leg work:

How to Leave the Catholic Church

AND

If you want to be excommunicated, you have to go through official channels. Your local priest can't help you; instead, you must write a letter to your bishop.

1. Tell him where and when you were baptized (they won't excommunicate non-Catholics).
2. 
   
3. Tell him of your apostasy; you must describe both an intention of apostasy and an outward manifestation. Apostasy doesn't count if you didn't intend it or if it didn't amount to anything.
4. 
   
5. Explain that you know this means excommunication—ignorance of the penalty would get you off.
6. 
   
7. State that you do not consider yourself a Catholic anymore and want your name taken off the official rolls of Catholics.

If you don't hear back after a little bit, send the letter again—but this time registered mail with a note that this is your second attempt. If you persist, you should be successful.