Tightening threaded headset locknut I'm adjusting the threaded headset on my Brompton (if that matters). I read Sheldon Brown's and Park Tool's articles on this topic and followed the recommendations. I adjust the upper race so that there is no play in the headset and handlebars flop to one side when I lift the front wheel holding the frame. The problem is, when I tighten the locknut with a wrench while holding the upper race with another wrench I still have the upper race move slightly. Thus, I get too stiff an adjustment -- handlebars do not flop to one side anymore and I feel the front wheel rotate tighter than it probably should. How do I properly tighten the locknut then?

P.S. For now, I have managed to tighten it somehow, with trial and error and then more trial and error. But still I believe I'm missing something here.
How do you get to Carnegie Hall? Practice, Practice, Practice. There is no magic bullet for dealing with this problem ( it's the same problem you'll have adjusting cones on a hub. ) Knowing just how loose to leave it so it's tightened correctly when you're done is something that comes with experience. 

This tweaking required to get it just right is one of the reasons threadless headsets were invented. They take far less labor and effort to install. 

One trick I've tried is to slightly overtighten and then use the lower wrench to tighten up against the locknut. Either way it helps if you keep pressure on both wrenches rather than trying to keep one still while tightening the other.