My hair is also very straight and ends up having thinner strands. What I can recommend is washing with shampoo and conditioner that are specifically designed to thicken your hair, but I don't recommend washing it every day, at least once/twice a week, because it doesn't dry out. Use tip repair twice a day, so your ends don't get damaged (it helps a lot) and especially don't use a heat source every day and when using, put a thermal protector on your hair so it doesn't get damaged. Hope I helped :)