Muffler problem

I have a 1990 Olds Cutlass Cierra with 150,000 + miles. Every year or two I end up having the muffler replaced, because it makes a horrible rattling/rumbling noise. I've been told the muffler's baffle plate is shot, and that it is a common problem in cars used for short distance, in-town driving. Because this has occurred so frequently, I am wondering if there is something else contributing to the problem. Any suggestions?