Are you currently at capacity?
To gain productivity, you must know your current capacity. "Capacity" is defined as the time allotted to work. Regardless of how much time that is, there is a finite amount of it and it's likely to stay within a certain range. If you know exactly what your time capacity is, you are more inclined to only accept new commitments if you have capacity to complete them. Knowing your true capacity at any given moment helps increase your ability to focus on completing things you've committed to.