Set aside some easy, short, fragmentable, fun, non-urgent, flexible, or low-importance tasks to do during “idle time” or “low motivation” moments.  These are especially good if they use different resources from the main task; for instance, if you plan to spend most of your day working on your computer, have a few pieces of low-priority paperwork to fill out for those times in which the computer is busy (e.g. rebooting, downloading, performing a computation, etc.).