Remember not to link your own name, address, etc. to these accounts by going online on these phones and logging into your own email, texting too many people known to be linked to you (since many databases now track affiliations to other people) and/or logging into your bank account, school website or corporate network. Maximum privacy means using cell phones primarily as phones.
This means when the iPhone comes out as a prepaid later this year, you probably don't want one. Too much temptation to give your true identity away by doing all of the things you'd do on your home computer on your iPhone. You still have to watch what you do on your cellphone as much as you do on your home computer, whether the phone's prepaid or not. One trip through customs with a phone where "questionable" content was viewed, received or downloaded and then discovered by customs officials will show you why. So, use ANY phone in your possession with care, prepaid or not or you'll destroy the very privacy you're trying to maintain.