Federal law allows you to inform your union of your intent NOT to be a member each year and receive a portion of your "lunch money/dues" back. Most unions require a new notice of your intent each year, to be received by them during a certain month, and the letter must contain certain info from you, otherwise, they can assume you want to automatically join back and they will restart collecting full membership level dues.
You would then be considered to be an Agency Fee Payor and not a union member. You won't get to vote on union meeting stuff, and they don't get to buy elections with your money. They are supposed to still be required to represent you in any/all contract negotiations and/or disputes you may have with your employer - hence the portion of dues your state won't allow you out of.
I tried this where I work. I didn't make many friends out of the deal, but I did save a bundle of money - more money than I could of saved if I'd switched to Geico. :biglaugh:
Try this site for lots of info on the subject
http://www.nrtw.org/rtws.htm