Be healthy. There is not a whole lot else you can do but I would suggest a very healthy life style with exercise, avoid toxins like smoking, and eat a good diet. The mediterranean diet is probably the best.
Many options. Endometriosis is generally treated with a combination of surgery and medicine. Surgery should be performed to make the diagnosis and remove as much of the lesions as possible followed by medication to atrophy any remaining lesions. Medications include birth control pills, progestins, nsaids, depo-lupron or danazol. Hysterectomy is the ultimate treatment once someone is finished with childbearing.