I've got a few older quill stem type bicycles I've went through and love riding. On two I can't seem to keep the headset tight. The one could probably use replacement parts, but the other everything is very nice. So what's the trick to keeping these things from loosening up? Am I not getting the locnut tight enough?

You are not getting it tight enough when you adjust it.
I expect you need proper headset wrenches.

a 32 for 1" 36 for 1.125"

the top nut of steel is a good move,
even in an aluminum headset, with steel races.

+1

Without two good wrenches you'll only fight with a threaded headset and still not get it as secure as you could with the proper tools. It's possible something is stripped out but I think you'd notice that.

it also has alot to do with technique too. you need to properly adjust the bearings and tighten the locknut against the upper adjusting cup or cone.

isn't 36 for 1 1/8" headsets and 40 or something other for the rarer 1 1/4" headset like the fisher what was it evalution?

Actually the wrench size it depends of the headset Specially in the 1 inch ones. Agree with the girl, it is like a technique. Basically u tight the headset with the bottom lock instead of the top one. So you have to figure it out how much to tight to top one and then lose the lower one so it doesnt get any play.

Tighten the main, large nut down onto the bearings, overtighten it a little, so the steering becomes a little stiff; then tighten down the locknut, smaller nut, until it reaches and tightens against the main large nut; Now, this is the important bit, Turn the main nut, large, anti-clockwise until it bites nice and tight against the small locknut, and relieving the load on the bearings to what should be optimum

Make sure you have enough spacer so the lock nut isn't bottoming out. I've often found this on bikes where the reflector mount was removed and had acted as a spacer.

Thanks for all the replies! I will give them a whirl, and also check if I need to cut up a reflector to act as a spacer, good thought.

Headset adjustments seem to be the most challenging for most new wrenches, or at least my experience in teaching classes has shown this.

It is not so much a matter of knowing theory as setting pre-load is the same as when you do hubs and bottom brackets but that the technique is a little trickier to make sure things stay where they have been set..

If it is coming loose after you tighten it down all the way, it could also be a matter of putting in an additional spacer.

I am going to put a spacer in it this week, and give it a go on the weekend. For now it's a different bike on rollers during the week. Thought I had it the last time, but it loosened up after 15 miles of chipseal and filled potholes. I think after reading this thread, it'll be dialed in soon

A threaded headset can be impossible to keep tight, if there's too much slop between the threaded cup (or cone, depending on the design) and the steerer. I've had this problem once or twice before with shagged-out ally ones; no matter how hard you crank it against the locknut, it's not enough to prevent road shock or maybe sprinting torque shifting the cup on the threads a fraction and starting off the loosening.

Can't remember if I tried fixing it with teflon tape; that might be worth a shot if this is your problem.

I have a campagnolo C-record in one of my bikes that practically was tighten using my hands. Never a problem.

If you are doing it the proper way with two wrenches and you cant get them tight then maybe you have some stripped threads on the nuts or steerer tubes