Here is a great post written by a University of Texas professor on why we need to stop negotiating against ourselves! I know most of us are guilty of it, we say "I can't get X because of Y".
Recently I rationalized that I couldn't land a large sales deal simply because the potential client was using one of our competitors. I assumed they were happy and had no desire to switch. It wasn't until much later that I found out that the potential client was in fact unhappy with their current provider and I swept in and landed the deal. While this example has a happy ending, if I would not have negotiated myself out of the deal up front then we'd have landed the business MUCH sooner.
Have a read...