A friend of mine used to keep an old piece of carpet rolled up in his garage for work like this - just unroll it and roll the bike up on it before you do the messy bits. The carpet absorbs all manner of sins and is easilly rolled up and put away. When it gets too nasty throw it out and get another piece - carpet shops are usually happy to give away old carpet that's been ripped up and replaced.
It also gives you a comfortable bit of ground to sit on while polishing spokes, cleaning chains etc. Flattened large cardboard boxes work well too.
