I agree, I think you are over cooking them. Also as TexasShadow suggests, use bone in, skin on breasts. I brine most chicken when I have the time. Depending on how many breasts you are going to roast, for two I would use 1/4 cup salt and 1/8 cup brown sugar dissolved in 2 qts. water. For 4, double the brine. Place breasts in a gallon size plastic bag, seal, place on a sheet pan and refrigerate at least 4 hrs. 8 hrs or overnight is better. Remove from brine, rinse, and pat thoroughly dry. Rub with olive oil. I don't salt if I brine more than 4 hrs. Pepper or season however you like. Let sit out at room temp for 30 minutes. Roast at 350 degrees for 30 minutes or until juices run clear. Remove from oven, tent with foil and let rest 10 minutes. You should have juicy, tender chicken breasts.