I came across this exercise to build muscle strength in the arms and shoulders.

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, 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 50-lb. potato
sacks, and 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 you feel confident at that level, put a potato in each
of the sacks; but be careful!