Originally Posted by Bryce Swartzwelder

If you manage to pull the bar ends off again, roll them on a known flat surface. The other fellows here may well be right that you have bent ends. Remember if you get clip ons that the stock ones come with holes for the bar weights etc and new ones won't have holes drilled for any of that.

If the tube is bent there is only one real solution that is to replace it.

If it is a matter of loose threads, You could try using a little blue loctite on the loose one if you dare, but it might be a devil later in life. Teflon tape is about a dollar and you can add it to the threads in layers. I would add two or so layers to the screw threads and see if that helps keep things in place. Its easy to remove and won't seize anything up. You find it in the plumbing section of your hardware store. Automotive stores carry it as well.