Walter, I'm in the same boat. We did a rolling exercise a few classes ago that I was terrible at, so I started working on them on my own. I can offer some helpful tips given to me by a few seniors and my instructor. This is for a roll from standing, btw.
-keep the back leg straight, this is easier if you do your best to watch it travel as you go through your roll
-katana arm is really important, I was letting my arm crumple, which led to pain in a few parts of my body and in my head
-don't curl in too far - initailly, try and make your hand contact the mat directly in front of your lead foot, not off to the side
-try and finish the roll sort of on your side so that the outside of your ankle is facing down towards the mat, not your heel, but don't let your ankle hit the mat (think about how much that would hurt on cement)
-try not to go outwards as much as upwards, the higher up you go, the more time you have to control your fall--when I go outwards, I find it harder to keep my arm from folding when my hand contacts the mat
The rest is all a circular blur to me, so I don't have much else to offer at this point. Best of luck.