Why do i keep getting re-occuring UTI's? (Urinary tract infections?)?

I keep getting UTI's i do everything to try deter them eg. urinating before and after . I shower 1 -2times a day and i follow what ever my doctor tells me to do to try and prevent it. My doc thinks i might have bladder or kidney cysts but i still have to go to get the ultrasound. any ideas?