At the height of your flare-up, you may need complete bed rest. Your body may not give you any choice. Even if you're weak, though, try not to stay in bed for more than a day or two. Spending too much time lying on the bed or sofa will make you stiff and increase your pain.
Once you start to feel a bit better, get up, stretch, go for a walk, and continue with as many of your usual activities as possible. You may need to cut back on your work, exercise, and social activities, but don't abandon them. Take breaks when you get tired.