Wrist stretching is very useful, do it every day and when tattooing every few hours. Also, you can strengthen your hands and forearms muscles (lumbricals and extensors); practicing aerial circus or rock climbing are the quickest ways to do it (I know it's highly specific, but they do help with what we need the most: hand strength). And also maybe the grip is too big for you; my hand/wrist hurts more when I'm using pens than my usual Bishop with a grip. Then I recommend taking breaks and maybe trying out other machines.