Along with breaks, I like to set myself small, achievable goals per day. Like, my pages are hand-drawn, take a LOT of work to color, and there's no undo button (yay traditional art...), so I really have to pay attention and be careful. So I'll set myself a reasonable goal per day, like 'ink that page', or 'color two panels'. If I'm not trying to do too much in a given day, I can concentrate on doing my best with the limited goals, instead of stressing about getting an impossible amount done.
And if I finish doing my goal, and want to keep going cuz I'm in the groove? Awesome. Bonus points. I just have to be careful to listen to my brain and body when it tells me it's done for the day, and that continuing will result in... shall we say, 'declining quality'.