First, try to straighten them with minimal damage – choose a straightening tool that works better for you: straightening brush, hot air brush or flat iron; select the right heat level for your hair type, and always use a heat protectant. Try to reduce usage of a blow-dryer and let your hair air dry when it’s possible. When it comes to styling, the less heat is always the better.
Second, use those hair straightening products that nourish your hair, moisturize it, and protect from heat damage. Choose them depending on your hair type. Moisturizing shampoos, hair masques, and argan oil (and other oils) will come in extremely handy, too.