Okay...this really works and will build up those shaky arm muscles in no time!
With a five pound potato bag in each hand, extend your arms out and hold them there for as long as you can. Do this until you can hold it for a minute with no problem.
Move on to a ten pound potato bag. It may take a couple of weeks to accomplish but the results are worth it.
After mastering that, try a twenty pound bag. Feel that burn? This is a good thing.
A hundred pound bag sounds impossible, but you can do it.
Once the hundred pound bag has become a piece of cake, add a potato. 
I know, how lame, but I thought of this old Internet post when I couldn't lift a bag of Purina Dog Chow this morning...I wonder if a 40 pound dog food bag would work! 
Huggies
Donna