The first thing I do before pulling my mac out of the sleeve or from the bookbag is wipe off the surface it is going onto. I know this sounds like I am a freak about my mac or should I say mac's, but once those scratches get in there it is hard to get them out.
The iBook would always loose a foot and turn itself into that rocking chair that you always get stuck in at the restuaraunt and you just can seem to quit rocking😀.
The only plus about your situtaion is that you dont have DENTS! My 12" powerbook has one tiny dent on the bottom, but my mbp has yet to see anything like that. You have the "durable" Mac.
So, I guess my advice would be to just keep it on a sleeve when you take it out, a book like someone else has mentioned or if it gets to you that bad, replace the bottom🙂.