When your PM is full and people complain, or BEFORE you lose messages, clean your PM periodically!
Consider cleaning your PM every time you clean your truck.

To clean you PM boxes, just follow the following instructions:
Be sure to clean all three boxes:
  • Inbox
  • Sent Items
  • Message Tracking

And select "Show Messages From the beginning" prior to cleaning out each box, as the normal view is only "last 30 days"!