I have arthritis and tendonitis in both hands, wrists, and arms. I have had to learn to loosen my grip on the needles and yarn, and let the yarn flow. I must knit slowly and carefully, and take breaks often.
Sometimes, I will switch from knitting to crochet, and back again. Embroidery is a nice change as well...especially in the summer.
Occasionally, I have to stop crafting for a day or two, and put ice packs on my arms and hands. Ibuprofen helps too.
But the thing that helps the most is just loosening my grip on my crafting tools. I keep reminding myself, "just let the yarn flow!"
Hope this helps!