Maintaining aspect ratio

Just bought a Toshiba 42RV530U LCD, I can't seem to get the picture settings to stay where I put them.
I set it the way I want, but when I change channels and go back, they change.
What am I missing?

Many HDTVs will turn off the zoom when they receive a HDTV signal, so if you change from a SD channel to a HD channel the zoom will be reset.