One thing I find helps with avoiding head and neck contact when rolling over backwards:
If, for example, starting with right foot forward, extend the left arm out to the side (palm down). Put the back of the right hand on your left cheek/ear. As your back leg collapses and you roll over your left shoulder, try to touch the mat with the palm of your right hand. In doing so, this should force the diagonal line of contact others have already mentioned, and protect your head at the same time.
This is so hard to explain in words! The only bit I feel adds to what others say is by using your hand like this, but the best advice I could give is to ask your sensei or a senior to help you.