a trick i used was folding up my spare flat sheets so they were the length of the bed, but only the width of the body groove. I would then slide then between the mattress and boxspring to fill in the extra space where the mattress had gone flat from the body grove. You could also use spare towels or throw blankets (like those fleece ones that there always seems to be an over abundance of)

Thanks for the Costco tip!

I know that not every Costco carries the same stuff and not everyone shares the glory of having a Costco nearby, but I found a reeeeally nice mattress cover there with memory foam (or whatever stuff that Tempur Pedic is) and a layer of down on top, and it was less than a hundred bucks.. It would practically make a cement floor comfortable, but I used it in my dorm and it was one of the best things I have ever done. Just thought I'd throw that out there. A really cheap mattress with a good cover can be a lot cheaper than a nice mattress!