The simple way is to just go work out.
Pushups, jumping jacks, crunches, etc
The other way, which I often use is to make a list of things I want to do, things I need to do, and things I want to accomplish.
I then start doing the list. This always gets me going. Now the best way is to start the list is prioritize it and start with the most important. I do not always do that. Sometimes I go for what I want to do and sometimes I go and complete the easiest.