Tightening threaded headset locknut
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.