Begin by standing on a comfortable surface, where you have plenty of room at each side. With a 5-lb 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 10-lb potato sacks.
Then try 50-lb potato sacks and then eventually try to get to where you can lift a 100-lb potato sack in each hand and hold your arms straight for more than a full minute. (After 6 months of work I'm proud to say I am at this level finally).
After you feel confident at that level, put a potato in each sack.