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