In school, I would always go to the end of the chapter and look at the difficult exercises, think about them for a while and fail to solve them. This gave me the motivation to read the chapter and do the easier tasks. It puts your mind in "the right state", knowing that you are working on a real problem.

Later in life I've discovered a trick that gets me into this state of mind more easily. For any feature, I ask myself: What problem does it solve?