Really it is much more an issue of conditioning than strength, and there are no short-cuts. The best thing you can di is just puppeteer for as long as you are able, make note of how long you lasted and aim for longer the next time.
I actually went for a couple of years without puppeteering and then was called to do a show that called for about 45 minutes of continuous puppeteering. I was surprised that I was able to do it, but a big part of it was that my arms "knew" what to expect.
Also, sore arms are an expected part of the craft! (to a degree...)