I came across this exercise suggested for seniors, to build muscle strength in the arms and shoulders. It seems so easy, so I thought I'd pass it on, not that any of us would qualify as a senior.
The article suggested doing it three days a week. 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 l 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 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.

Once you feel confident at that level, "put a potato in each of the sacks."