Pole Vault help please!?

i currently vault 13 feet, and i am looking to try and get 14 this year.. and hopefully 15 next year, i am a junior and weigh about 135 and i am on a 145 14' pacer. my plant and approach are fine but it goes downhill from there. what are some tips anyone has to help me with my rockback (which is decent) and more importantly get good inversion so i can push myself off the pole. ne more thing how high would i need to vault to be able to in college?