i've had a problem with this, which seems simple but it didn't initially occur to me...

my cog was slipping a little, when i pedal hard forward, it would slip, then "lock" against the hub. when i would slow down/skid, it would slip backwards, and "lock" against the lockring. obviously i was like "well **** i need to tighten it down" so when i got home, i tried to tighten down the cog and lockring, but everything seemed snug. next day, same problem. then it occurred to me what was going on. i was coming to a stop before getting off my bike (duh) and in doing so, locking my cog against my lockring. neither wanted to budge under the force of my tools. so i got on the bike, took a good hard pedal forward (anticipating a slip). cog slips forward. got off the bike, then tightened the lockring. haven't had a problem since then.

don't underestimate the power of your legs over your arms!