This may sound strange, but I once loosened a stubborn lockring by slightly tightening the BB cup it was attached to. The theory is, as you're tightening the BB cup, the lockring stays flush against the BB shell and backs off the threads a little bit. Worked for me. You could also spray some WD, tri-flow, or liquid wrench on there as well, just to facilitate the process. Though it wasn't necessary in my case.

Big ol screwdriver and a hammer works everytime, but may hurt the lockring in the process.