This was posted on another forum i'm a member of, it sums it up nicely, though i did not write it:

this technique works best with a 5 speed suntour perfect freewheel(very common). instead of 2 chain whips which is actually pretty hard to pull off, use the chain on your bike and a chain-whip.
first take the freewheel that your going to separate on a wheel that fits on your bike. now tighten the bolts to hold your wheel in place than take the chain off off your front chain ring so the chain is sitting on the bottom bracket.
wrap the chain around the 1st or second gear cog so the chain is able to wrap over its self. this should immobilize the cogs from going any where if you do this right you can take the chain whip and put it on the outermost cog and start trying to unthread the cog(i find it easiest to use the frame and the handle of the chain whip and squeeze hard it takes alot of force to get the first one broken. then just unscrew the first two cogs and as stated before the rest of the cogs just slide off. this procedure works best for 120 spaced frames (but if steel you can always coldset) then you don't have to redish the wheel. when you thread the first 2 cogs back on just one will work if you want (i like 2) just hand tighten them for when you start pedaling you will tighten the cog on the more you pedal.