No matter how effective you are, you can’t fit 100 hours of work into 40 or even 60 hours a week. Rather than letting a vague sense of fear drive your decisions, take an objective approach. Gather your facts: a concise list of projects, an estimate of how long each task will take, and a visual to show the incongruence between the available time and the requested activities. This visual can be as simple as a printout of your weekly calendar or as complex as a full-scale project plan. Then ask for a strategic planning session with the people who are asking for the most work from you. Discuss which tasks might be delegated or simplified so that you can invest more time in the highest priorities. When done in this manner, asking for direction can lead to a joint effort to prioritize and work within the reality of your schedule.
Source: Harvard Business Publishing : Today's Management Tip was adapted from “Stop Work Overload By Setting These Boundaries” by Elizabeth Grace Saunders.