In his Sunday Planning System, Darren Hardy talks about saying no to everything that does not fit into his three big goals. Protecting your most precious asset, your time, is the truest testament to discipline.
There is a difference between understanding that life happens versus letting someone's lack of organization and poor self-management affect your results. Know when to negotiate and when to stand your ground. You might get some resistance, and it will not feel good. Do not cave in. People will keep testing the waters each time you let things slide and look for ways to get you to agree to their demands. Your time is valuable, and there is a price to pay for those who wish to enter your space.