I've been training to arm wrestle Witney, my personal trainer has given me this regime to follow:
Begin by standing on a comfortable surface, where you have plenty of room at each side.
With a 2 kg potato sack in each hand, extend your arms straight out from your sides and hold them there as long as you can. Try to reach a full minute, and then relax..
Each day you'll find that you can hold this position for just a bit longer. After a couple of weeks, move up to 5 kg potato sacks.
Then try 25 kg potato sacks and then eventually try to get to whereyou can lift a 50 kg potato sack in each hand and hold your arms straight for more than a full minute. (I'm at this level BTW)
After you feel confident at that level, put a potato in each of the sacks.