A table saw or other machined surface is excellent for keeping sandpaper flat for the "scary sharp" sandpaper sharpening method - which is what I use most. However I like to be able to tape the 'base' sandpaper sheet to keep it from moving around. I lay the finer grits on top of that to keep then from moving as well.

I went to a local tile shop and asked about 12x12 marble tiles. When the woman at the counter found out I only wanted a couple she pointed me to a pile in the back corner of their yard where they had some odd lots and remainders and told me to help myself. Now I just grab a tile, lay the appropriate grit on it, and touch up whatever blade displeases me. Not a major deal, but the price was right and I am more likely to use it because it doesn't disrupt anything else I may have in process.