"Help yourself to" means something is freely given to you, but you have to take it yourself. It could be food or drink ("help yourself to some coffee", "help yourself to a bottle of water", etc.) or it could be something else ("help yourself to a newspaper" or "help yourself to a free book"). It's commonly used. "Help yourself with" is not as common. I think it can be used in a "self-help" context. "Help yourself lose weight with our new diet" or "Help yourself overcome anxiety with bio-feedback".