I use a chainwhip to get the cog really tight, then sprint up a hill and use my brake to slow down, then rotafix it tighter. My theory is that the rotafix will make it tighter than I would tighten it on a hill, so it's not going to move any further. You could easily do the initial chainwhip bit with rotafix, so just rotafix pretty tight, sprint up a hill, then rotafix a bit tighter. Then snug the lockring.