Always deliver on your promises. And, whenever possible, over-deliver. Why? Because everyone else will be doing just enough to not suck, while you are being a rockstar when you follow this tip!
Don't you want people to know that your level of professionalism knows no bounds? Whether you are faced with deadlines that were impossible to meet or changing priorities. No one cares that you had your back against the wall when you don't deliver... they only know that you can't get it done. Otherwise, you will have a following of people that have nothing but bad things to say about you. No one needs that!
Follow through says that you take a project seriously and finish it. That you can be trusted. When I take on a project, I break it into milestones and set deadlines for each part adjusting as necessary as long as my final deliverables are ready on time. This way, I always know that I am on tract for success. It feels much better to give someone a completed project early than an excuse to why it's not quite done.
If is seems that you will need a miracle to get the job done, dig deep and find your super power. Your clients will thank you!