What to do to keep yourself moving?

Some days I come into work and it's like I am on fire, but then some days I will sit at my computer at just stare at it for hours and accomplish nothing. Then I feel terrible because I know the only way to grow my business is to work on it but it's like I have a mental block stopping me some days.

Does anyone else experience this? What can I do to motivate myself to push through and still be productive on these off days?