I wrap a towel around the top tube to protect it.

I use a headset wrench for the race bearing, and a large socket for the top nut. I make sure all residual grease from the repacking is wiped off from the top headset threads, the top of the upper race nut, the washer, and that the locknut is free of grease, too. I want a good friction grip there.

Once I have the adjustment the way I want it, I try to turn the fork until I can put the headset wrench on, secure it along the top tube with duct tape wrapped around it and the top tube. Just so it's easier to deal with. Then, I overtighten the headset just a hair.

Then I put the socket down over the top and, while holding the headset wrench along the top tube, crank the lock nut down hard. Very hard, making sure any movement of the headset wrench is towards the handle of the socket wrench (counterclockwise). It often moves a bit while I do this, which "undoes" the overtightening I just did, giving me the play I want.

Seems elaborate, but takes just an extra couple of minutes, and less time than it would take to go back and do it again.