use the wheel and the chain to pull the cog off... sometimes chainwhips suck to pull off a cog. take the chain off the chainring and drape it over the bb.
put the rear wheel on and tighten the axle bolts so the wheel wont move out of the dropouts. put the chain on the cog and double it over itself until the chain doesnt spin and there is no slack between the cog and the bb. make sure you turn it the right direction to get the cog off. grab the tire and keep the bike stable and just turn the wheel like a steering wheel. the cog will spin off. you can also do this in the reverse process to put the new cog on...
http://fixedgearbikes.blogspot.com/2...-rotafixa.html