If you have a large or full-length mirror somewhere around your house I'd suggest taking a minute or two and try to see yourself doing this animation. You might notice that when you do, you personally won't look nearly as stiff and inorganic. Try to figure out why that is; how does your torso swing when you raise your arm like that? What happens to your other arm? Do you adjust your weight on one/both of your feet?
If there's no mirror available, try looking up some reference material
here. When you find something that fits your style, play close attention to the changes that the spriter made in each frame of the animation, and how the body moves to adjust to the new position.
I feel your pain, I'm no good at this either, but after a while of tweaking I can usually get it to look acceptable enough. Good luck!