How do you keep water from going up your nose for backstroke?

I'm not talking about the stroke itself as much as I'm referring to the starts and turns, I try blowing out bubbles, but I either run out of air to blow out of my nose before my breakout or water goes up it and I start choking. Please help, cuz I'm an Individual Medley swimmer and I just finished learning how to not use nose clips on fly, breast, and free, back is the only stroke I have trouble with.