It's tricky, yeah! Gosh knows I spend most of my drawing time curled up like some kind of shrimp. Sitting up straight does wonders!
If you are a twitter-user, I recommend following @tinycarebot - it's a bot that tweets tiny self-care reminders once an hour. Things like remembering to stretch, remembering to drink some water, reminding you to check your posture, to eat, to say hi to a friend, to take a break, etc., etc. Every single time it's tweeted "please check your posture", I've been sitting weirdly, so it's definitely been helpful for me!