Being willing to say no, is also a huge part of this. The sad thing is always saying "yes" often results in both a worse product and spending your life in the office.
I've also found that it can also damage your career. Being a yes-man does not gain you respect or the time to do well on what you take on. Not to be construed as "being difficult to work with is a good career move", it's just worth noting that saying no at the right time can be just as good a move as saying yes at the right time.