Take 10 minute breaks at least every hour and a half. It seems annoying but you'll be able to draw for much longer if you do. Don't draw through the pain. It WILL get worse. Make sure your drawing on a surface that's tilted towards you (like with a drawing table) so you aren't putting unnecessary strain on your shoulders. It sounds dumb but you can take a 5 lb weight and just do reps every now and then to keep your drawing arm strong. I started doing that and my wrist pain is non existent now. Also you can hold the weight in your hand and just move your wrist around as much as you can within a natural range of motion and that helps too.

Lastly if you still need help with pain after drawing you can stick your hand and wrist in hot and cold water alternating between the two. If you have a double sink you can fill one side with ice water and the other side with hot water and just switch between the two. Read this is how massage therapists help their hands and wrists and I gotta say it helps. This is really good if you're taking a long break.

But the increased focus on regular reps and taking breaks is what has really helped me the most.

Yeah. Only usually it's after 5 + hours for me. Anyway... what you need to do as Erik says is stop and take a break from the work for a bit. (Honestly, this is never a bad thing - the artist needs to take a break, stand up and stretch, take a bit of time away from the work and come back to look at it with 'fresh eyes' periodically, anyway.) If the hand is really sore, then I suggest you soak it in hot water with Epsom salts, probably 15-30 minutes until the water cools down. If it's just the fingers that are bothering you, then you are probably holding the pencil / pen / brush too tightly, and not knowing how you hold the items, I can't really suggest how to adjust it - we all hold tools at different angles and the type of pencil and the type of lead used makes a noticeable difference in the amount of pressure one applies, the type of brush makes a difference, etc. You may want to experiment with different kinds of tools and see if that makes a difference.

If it's your wrist and the entire hand, then you may want to invest is a light wrist guard or a wrap, that you should be able to get reasonably cheap at a pharmacy or Wal-Mart or something without involving a medical professional.

I just shake it out and continue but if that doesn't work--take a break and do something else. It's not worth carpal tunnel.

I just shake it out and continue but if that doesn't work--take a break and do something else. It's not worth carpal tunnel.

This all sounds like a lot of work.

...because I don't hesitate to erase or make changes.