Is it a converted road hub or a "real" track/fixed gear hub? My normal method for cog/lockring installation on track hubs is to put a little anti-sieze on the threads, thread on the cog and tighten with a chainwip, thread on the lockring and tighten with a spanner, then take it out for a brief ride, using only the brake to stop (or my feet if there isn't a front brake) while not applying any backpressure. After a few minutes of pseudo-sprinting, the cog will be tightened as tight as it will ever go, I think crank the lockring down again and the setup is good to go.

I've found that if I skip the "riding around" step, I can get some play in the cog/lockring combo that will allow my cog to thread and unthread a little bit. I like the idea of jamming a spacer in there a little more than this, but a little less than just getting the parts you have to interact properly.