Again, you don't *need* a chainwhip, especially since you can make your own out of spare chain. But the rotafixing technique works great. You basically let the chain sit around the bottom bracket shell and the cog (hand tightened), then double up the chain by the cog and turn the wheel to tighten:

http://204.73.203.34/fisso/eng/schpignonestep2.htm