Conserve your hand strength and activity as if it was money, something to budget. It requires more planning of your time and activities, but if you plan what the activities are, what they cost in terms of hand usage, and how much time you need to have between them, you'll find that you have more options. Take breaks - often. That could mean every 15 minutes if you are hurting. Breaks don't need to be long, but you want to release yourself from the static nature of what you are doing.
Variety is extremely beneficial.
Stand up, walk across the room and back. That's enough if you do it every 10 minutes of a taxing activity. Yawn a few times - or take a few deep breaths if you don't yawn on cue. Use your body gently. Experiment to find the minimum force required to do any task, whether it is writing, driving, or holding a paper.