Stand in a comfortable position with a 5-lb potato sack in each hand. Then extend your arms out from your sides for as long as you can. Try to go a full minute and then relax. Each day you will get a bit stronger. After a few weeks, move up to 10-lb potato sacks. Then try 50-lb potato sacks, and eventually try to get to to where you can lift a 100-lb potato sack 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 sack.
LOL,It's tough getting old...