Go into your Private Messages
Click the box to the right of the ones you wish to delete
at the bottom where it says 'Selected Messages:' select delete from the drop down and press 'go' button
Or you can upgrade your membership for $350 i think and your PM box wil l be raised to 900
You have to make a decision each time you send or receive an PM whether it is one worth keeping, then delete them as you go. This is what I *try to* do, as it is much easier than going back through 500 messages!