The key to maintaining straight hair is to use a good serum that protects hair, particularly one that contains silicone. Silicone is water resistant and prevents humidity from entering the hair shaft, which causes hair to swell and results in frizz. You should try to look for a shampoo and conditioner that also contain silicone, in order to further prevent frizz.
It is also essential to blow dry your hair prior to going outside if you want to keep hair straight in humidity. The reason for this is because if you go outside with wet hair, your hair is going to soak up extra moisture in the shaft, causing the strands to swell and causing more frizz. Once you have applied your silicone serum for protection, blow dry hair on a medium heat setting with a round brush, then finish off with a flat iron. Apply a sealing spray to finish off and hold the look.
Find out more tips here! Good luck!