We recently bought a memory foam topper for our spare bed. I think we bought it off Deals Direct for $200. It had to off-gas for a while. It's made that bed so much more comfortable. It's nothing compared to our main Tempur mattress, but at least we dont end up in agony if one of us sleeps in the spare bed anymore.

The topper we bought is about 10cm thick and is like a fitted sheet, so it's less likely to slip off the mattress.