That is when when you reward yourself not just for completing particularly arduous tasks, but while doing them.
HBR recommend doing something fun such as listening to music or working in a Wi-Fi enabled restaurant while finishing tasks, or meeting a friend so you can plough through your individual to-do lists together.
Other tips from HBR include giving yourself regenerative rewards such as doing physical exercise, talking to a good friend for a short time or getting a snack or drink after crossing something off your to-list. Productive rewards include taking meetings to restaurants as a reward for doing something dull, cleaning your desk, or reading a popular business article. Finally, you can give yourself a financial reward such as a gift card for completing big tasks.