This doesnt apply to me, but this is what a friend does and I like the approach.
They have unlimited text msg plan for an extra 10$ a month that the kids split and pay themselves. They also pay a portion of the cell bill, like 20 a month I think.
The phones dont go to school with them, just to make sure there are no issues with them getting into trouble with it, as cell phones are not allowed to be in use at their school.
They 'spot check' the messages. They dont go snoping, but sporadically will announce that they are having a spot check. They do this when the kids are at home and right in front of them. They check their phones, the numbers they are calling, their texts, emails, myspace and other type sites, other computer files, purse/backpack search, and sometimes room searches and reading hand written notes and diaries. They usually dont go through checking all of that, but will mix it up... this time it might be the computer and the phone, next time it might be the purse phone and backpack, ect...
They do it sometimes if they suspect something is going on, sometimes just completly random. Sometimes they go a while without any checks, sometimes they have one every day.
I like that approach because there is not any going behind backs and snooping which in my opinion causes a lack of trust in the parents from the kids. But at the same time, they still keep check on whats going on.
Some people would say thats a bad approach as the kids know its a possibility and simply wouldnt have anything incriminating. Not true. They have caught their children up in a lot of misbehavior that way. nothing serious, typical teenage stuff, but..