Calcium !
Drink, eat, do everything calcium 
And of corse all your other vitamins.

For building up muscle in your arm again, those power balls are great, Do the whole of your wrist, arm, sholder.
http://www.powerballs.com/

If you see on that page theres a link to "rehibilitation"
That will explain to you how it helps that.

 

I'm in a similar position mate, but my cast came off 3 days ago.

For the last 3 weeks, i used my arm as i would normally (as well as i could, minus riding, swimming and the other obvious things). But working, trail building, and lifting things seems to have helped no end.
The physio was very suprised as to the strength i had when the cast came off.
Now the cast is off, i have to go easier though, just making sure i dont put any great shocks through it.

Keep using it til you have an ache or pain which continues for more than a couple of seconds. Then don't push it anymore.

w*nk more 

Pomaz, wanking with a cast on isn't comfy


I'm still recovering from my broken arm, did it over 2 months ago now, got a metal plate on the Ulna and 2 on the Radius, with 5 pins in each plate. So I have a terminator forearm.

I can't ride for atleast another 6 weeks and will have to have another operation.

I have been using a power ball and also the forearm grips you get from Argos to increase forearm strength.

I have been eating loads of nuts They taste good and are full of calcium. Best thing is to have a fruit and nut cereal, as it means you have the calcium from the nuts and the milk.(this could not sound any worse)

As I can't ride I've now taken up golf as something to do again, I used to play ages ago, just got back into it. It's a laugh with mates and you can take your anger out on the ball!

I've broken my arm twice, I've become quite the left handed master.