We have learned that one of the most effective ways to reenergize ourselves is to change the activity. Perhaps, you are writing a report at work and you feel like you are losing focus. Then choose to work on a different project to regain your focus and with your focus comes your renewed energy. Then after you work on the new project for a while move back to the old one with renewed energy.
Tip: a great way to keep your energy high is to pre-plan to change the activity by setting an alarm to remind you to change paths. If you want a funny looking alarm get one like this. It is a very innovative device.