Hands are some of the hardest things to draw right and they're also something that a lot of artists don't bother learning well enough. I have put a lot of effort into getting hands right. Here are some of the things I've done:
1. I have a large filing system with literally thousands of clippings of cars, buildings, faces, and yes hands that I have clipped from magazines and newspaper though the years. I tape the clippings onto a sheet of paper and put that in a well-organized folder. Whenever I have trouble, for example, drawing a hand holding a telephone receiver, I flip to the four pages of clippings I have for hands holding phones in various positions.
2. I also recently hired a photographer friend of mine to take a couple of hundred photos of a man's hands and then a woman's hands in all manner of poses. I have put these photos in their own folder and they are now my first stop when I have a problem drawing hands.
3. On top of that, years ago, I spent quite a few hours going through my favorite Neal Adams-drawn comics and, with tracing paper, tracing all his best hands. (He is the best artist and he always took special care drawing hands--probably from his own photo reference files). The act of tracing helped put a lot of that hand anatomy in my head. Now, I would probably more recommend tracing the hands in my photo clippings instead of another artist's interpretation of reality.
4. Of course, practice sketch (with your clippings in front of you) whenever you can.
So, bascially, my advice is (1) collect clippings, (2) practice by tracing, and (3) practice by sketching/copying. Soon, you will be an expert on drawing hands!