Take the time to let the image form clearly in your mind. You may have difficulty with this at first, but persevere. Imagination can and should help you stretch those boundaries. I try not to censor anything during this play period. I just keep working with the possiblties until everything falls into place.
Flexibility here is important.
At times, I've started out with one idea and ended up with something completely different. This is one of my favorite approaches to beginning a new piece. Finding connections between objects that seem totally unrelated is a real imagination-stretcher. If you're dealing with ordinary subject matter, one way to make it more unusual is to imagine it in terms of opposites. This process is frequently expressed in art as abstraction and arbitrary color.