I was doing "kick-backs" (tricep extensions) like crazy until I realized it was just working the BACK of my arm and not the underside, so-o-o
I now use a dumbell that I hold verticially with both hands. I lift it over my head and drop it down behind my head as far as I can reach, then raise it up to just above my head and then back down again. I do four sets of eight: two with my right hand on top and two with my left hand on top.
You CAN just hold the dumbbell with one arm and do each set one hand at a time (but I find I can use a bigger dumbbell using both hands).
This is a bugger of an area to hit, but this one really works for me... Good luck!