I was brought up that once you have made a commitment to something, I am to stick to it. Yes, there will and has been times when I really couldn't stick to it (like surgery), but those have been exceptional circumstances.
With that attitude, I know I have sometimes taken on more than I should or realised it's all happening close together, rather than a bit more spread out. Unless I have really messed up and double booked myself (which, fortunately, has happened once and turned out the time of one thing had changed from what I was originally told) I'll get on with it.
I also won't take on something which is at the same time as something I have already committed to. The other thing may seem more interesting or useful or any number of other excuses why the original commitment could be bumped. I think in ministry this is especially important, as a meeting with a member of the congregation or colleague may not seem as important as a training course or phone call, but it's important to the person I am to see. After all, I am called to serve and if I just change meetings around due to more interesting (or seemingly important) things coming along, I am being self-serving.
I know there are times when things will need to be re-arranged, but I'd like to think they'd be the exception and not the norm. In fact I sincerely pray they would be the exception and I always focus on who I am serving.