There can be many tips to grow your hair longer. For a start, if you want to grow your hair longer you should only trim them (when it becomes absolutely necessary).
You should also oil your hair daily and massage your scalp. Choose the oil that suits you the most.
You should comb them twice a day and avoid hair coloring and perming because this could damage your hair.
You should eat healthy food. This could increase the hair growth.
Avoid dying, curlers and straighteners which could harm your hair.