The comfort zone has been described as a type of mental conditioning that causes (or allows) us to operate within fixed boundaries.
We all have comfort zones but successful people tend to be more willing (or able) to step out of them. I like the idea that the comfort zone is a mental condition because it implies that we can modify, expand or even completely circumvent them simply by deciding to do so. Of course, tight economic times often require that we do this even if it's not something that we would do otherwise.
One of the easiest ways to get out of your comfort zone is to take the advice of the Seinfeld character George Costanza and simply "do the opposite." I was told once by someone I respect very much that we can only have what we can imagine. These two ideas relate to each other in the sense that we can compel ourselves to do things that we may not normally be comfortable with. Picking up the phone and cold-calling possible sources of work, for example.
Another friend used to talk about the law of mental inertia. It is the same as the physical law of inertia which states that an object in motion remains in motion until acted upon by an outside force. Our thinking tends to continue in the same direction unless it is altered by an outside force. The main difference is that we can provide the outside source for within by changing the way we think or by imagining a different path.