Dog doesn't listen when we're outside. How do I get her to focus on me?

My dog is like night and day inside and outside. Inside, she is very attentive, follows me everywhere and watches every move I make. She is about to finish advanced obedience cl and is doing fantastic. However, once we go outside she turns into what I like to call the "beast." All of her natural instincts take over and I'm lucky to get her to listen to me once out of every ten commands. It's like she doesn't even hear me. She is just more interested in sniffing and pulling and trying to chase squirrels. She's a very strong dog too...so she can literally drag me. Now, we have mostly fixed this problem on walks with a choke collar and training (suggested by our obedience teacher), but now we are doing agility training and I have a hard time keeping her focused and behaved while the other dogs are having their turns on the equipment. What kinds of things can I do to get her to focus her attention on me more when we are outside? I've tried treats...she doesn't even care about them when she is outside...