For the point[s] where there is contact, clean the surfaces well and rough them up a bit with coarse sandpaper. Will not fix your problem, but in combination with what others have mentioned, it will help.
Cut another notch into the pipe on the end that clamps to the head. It might just be that your pipe isn't able to close down enough to really grip as well as it should. If I remember correctly, my pipes have 4 notches on them, and I haven't had a problem with them yet. I know some aftermarkets have only 2-3 cut in them. If this isn't your situation then I guess this idea won't be worth a hill of beans to you.