had the same thing happening and i couldn't figure out why the lockring wasn't holding the cog tight enough, especially since the guy at my lbs tightened it right in front of me. ...but it was still slipping. found out it wasn't being tightened sufficiently because he was tightening the lockring without removing the chain, therefore the spanner was not straight on the lockring because the chain was in the way. if the spanner isn't perfectly straight on the lockring you can only get it so tight before the spanner pops off.

make sure you do it right. crank hard up a hill, get off your bike. then remove the chain and tighten that lockring down as tight as you possibly can. should do the trick.