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, 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 10-lb potato sacks.

Then try 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. (I'm at  this level.)
   After you feel  confident at that level, put a potato in each  sack.

Thanks to Matt for this exercise program.