Motivation is needed for something towards which we are not naturally inclined, a subject that we approach not out of love but instead out of fear or greed.
An artist, working in a corporate setting, would need to motivate herself in order to attend effectively to her job because she is afraid she will lose the job otherwise and not be able to pay her bills.
Similarly, a person may get caught up in a wrong profession or pursue a lifestyle unsuited to her in order to get what this alternate way of living has to offer (more money, fame, popularity etc).
However, when we are in our element and doing that which is our original nature, then motivation does not even come into question. No one need motivate a child to play. No one needed to motivate Mozart to write music or motivate Einstein to attend to the secrets of this Universe.
Hence, when you feel you are in need of motivation to do something, know that it is not part of your nature and will not bring fulfillment in the long run.
This however, is not a recipe for sloth, lethargy, or laziness. How to know the difference? Your sense of inner joy and well being is the true indicator. If you are feeling lethargic, sad, and as if you are only going through the motions of life, know that you have stepped away from your calling in life and are chasing a dream that is not your own.