I hate having to my dog. Please help?

My chihuahua is a great dog. She doesn't yap, she's usually relaxing. I take her to the dog park once a day. I play with her a lot and I allow her to bite, when I am finished playing I (literally) tap her on the head and say "No. I'm done playing." very sternly. Sometimes she gets riled up for no reason and attacked my feet and biting them. When I say no, sometimes she won't listen so I say, "You're going to get ed!" If that doesn't work I give her a swat on the , that usually works. The thing is, it hurts me that I have to her. I've tried (as the Dog Whisperer recommends) pressing my two fingers to her neck, but it doesn't work. How can I discipline my dog in a non-painful/affective way?