if you have a vise, put a block of wood on each jaw to protect the teeth of the cog, and spin the rim to tighten the cog down.
if you don't mind the possibility of scratches, you could use the italian track method. described here:
http://204.73.203.34/fisso/eng/schpignone.htm
you could try padding the BB shell with a rag or such.
and the riding method bostonfixed described works as well, just depends on how much time and effort you want to put into it i guess.