When you find yourself doing something, notice where the motivation is coming from. Are you doing it to please someone else? Or is it for yourself?
If the answer is that it’s motivated by pleasing others, then ask yourself why they want this thing done. Do they really need your help? And how will helping them make your life better?
If the task being asked of you doesn’t fall under one of those categories, if it doesn’t help anyone and doesn’t benefit anyone (including yourself), then try to find an alternative way to say no so that the other person isn’t put out or upset by your decision not to do what he or she wants done.
