The most common cause of hair thinning is aging with family history(heredatory).If this has happened in your family, then this problem will increase with age.In males thinning of hair also more common due to androgen,which make shrinking of hair follicle and hair loss.Generally these conditions cannot be prevented but can be controlled.Among preventable causes most important is diet and another one is stress.Protein,Zinc and Iron are essential for hair with vitamines like biotin.So eat egg,legumes,fresh vegetables and milk with plenty of water.There is drug named minoxidil,which may be used.Another option is surgery in the form of transplantation,which is little bit costly.Do not wash your hair more than 3 times in a week. If you use any shampoo, use only a mild shampoo.Do not comb the hair backwards. Use a good quality brush/comb. Avoid excessive brushing.Massage the scalp vigorously for 10 to 15 minutes (until you start to feel hot there) after bathing it with water. This stimulates the blood circulation and strengthens the hair follicles in the scalp.Coconut oil and castor are very good for hair.