Chicken breast rubbery inside

When I bake chicken breasts, I can't seem to get them to come out how I need to. Their either way too tough, or disgusting and rubbery/slimy on the inside which is even worse. I'm not sure what time/temperature I need to be cooking them at. Normally, I cook them in the oven at 350 degrees for 35 minutes. Sometimes I'll cut them up into small pieces and pan sear them, in which case they turn out better but preparing them like that takes a lot more time. I try to save as much time as possible due to the fact that I'm always on the go and need to make things as efficient as possible. What I was wondering was...is there anything I'm doing wrong as far as the baking that would make the chicken come out slimy and rubbery on the inside? I figure it's undercooking, but I'm not really sure. Should I try to cook on higher heat for less time, or lower heat for longer? Any help is appreciated!