This may be a bit contentious, but I would find a dvd of basic ukemi (falling) training and practice on the lawn. Even if you just do some basic exercises such as rolling around on the ground without falling down or standing up it would be helpful. Not sure what is a good dvd to use though.
Yes, practising rolls before you begin will give you a good head start. But start slowly and carefully from a seated position. It should be painless and you don't want to injure yourself. Practise on a soft surface.
There are lots of instructional videos about basic ukemi on youtube, I more or less randomly picked some.
These are instructional videos about rolling from jiujitsu, but we practise it the same way in our aikido classes:
-Back roll: http://www.youtube.com/watch?v=Rz8lx969WXs
-Forward roll: http://www.youtube.com/watch?v=W93w6aw26Ls
At 0:43 http://www.youtube.com/watch?v=7wV49Q9mpUI
shows a variation of the forward roll from seated position (the front arm going outside instead of inside). (BTW, the instructor in this video is also a member of AikiWeb, Chris Hein)
It changes which part of the shoulder you roll over. You can try and see which way feels the most comfortable for your shoulder.
With back rolls you can also move the arm out like that, as in http://www.youtube.com/watch?v=tXBlZS6U6ns
starting at 1:20 (the part before 1:14 shows what you shouldn't do).