My job is to be helpful, customer oriented, resourceful and my bosses right arm.  I have learned over the years how to pace myself and not set myself up to be sucked into stuff.  I do go out of my way, when I can.  I do put the customer first, both internal and external.  However, I do not let people walk over me and have learned how to politely tell them "no" when I know I will be stretched too thin and not meet the expectations of my boss and the objective of my job.  It is a thin line we walk.  It is all in how you tell them no that matters.