I go for walks and let my mind wander aimlessly for an hour or so, heading back when the thoughts in my head finally start turning towards work related stuff and I want to engage with it.

I also tend to keep a few tasks on top of the stack and try to keep enough room before deadlines to let my self switch to something else when I feel blocked or unmotivated.

Also helps having a boss that is well aware of ADD tendencies and can work with them.