I have a WTB grease guard threadless headset on my bike. It seems to need a whole lot of tightening to get rid of the looseness and even then seems to loosen up during rides. Normal commute riding.
Yes, I do loosen up the stem bolts before tightening the star-fangled nut.
Yes, the problem is in part the flexy front fork.
No, the steerer tube doesn't seem too long.
Other than those obvious do you have any other tips. Is it just me not being used to threadless and not tightening it enough?