How to Thicken Hair. Ingredients Needed: 1/2 cup mayonnaise 1 tbsp castor oil 1 egg Directions: In a bowl, mix all ingredients together. Apply to damp hair and cover with a hot, wet towel. Leave on for 30-40 minutes and rinse out. Shampoo and condition as normal. Repeat 2-3 times a week for at least a month. Repeat once a week thereafter.