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 15-lb. potato sacks, and then eventually try to get to where you can lift a 25-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.