I'm sure many of you have done this, but for those that haven't,here's a small article I wrote up on how. I took advice from alocal 240sx Club member and decided to try it myself oneSunday afternoon.
Basically it involves adjusting the existing side bolsters of theS13 cloth seats, so it hugs you more.
As ghetto as it may look, it works. Here's the article (which contains the pics):
http://240sx.cubicdesign.com/article2.php