I used to capo my guitar by using a pencil (broken to be just a little longer than the fretboard is wide) and a lot of stout rubber bands. Pain in the ass to get on but once on it worked as well as any capo I've ever used.

Other things to use when your slide is in absentia:

1) Bic lighter
2) Swiss Army knives (the body)
3) Beer bottle (other bottles work but there's something about used alcohol containers and slide guitar : )