I really like the Hozan lockring tool I picked up - way more torgue and less slippage than the Park tools. No photo, but here it is for 19.95 at Harris :
http://harriscyclery.net/site/itemdetails.cfm?ID=530
Chainwhips are totally neccessary, but you might have to modify it. Most of the ones I've seen came with a 3/32 chain, which I had to change to 1/8 in order to really lock the cog down.