My hair gets greasy extremely fast!?!? HELP!!?

I have extremely straight dirty blonde hair, so straight people think i straighten it and r completely shocked when i tell them i don't. i have to shower every night or if i have athletics that day, people think my hair is wet! so pretty much, straight hair, athletics (they over work us i swear lol) and i have to shower every night to keep my hair from looking WET!? >=