How do I stop chicken breast from drying out in a Casserole?

I usually use chicken thighs when I cook, but I am on a diet and have switched to breasts instead... but... they always seem to be dry when I use them in casseroles or simmered in sauce. I went to a fair last night and had the most amazing chicken and mushroom casserole, and the breasts were so succulent and juicy! What am I doing wrong?? I usually brown the chicken a little, then add the sauces and simmer on low. I have tried decreasing the cooking time, but I still end up with dry breasts and a sauce that is too runny. Thanks for any advice!