Generally I think about flow in terms of setting up intermittent reward cycles where I need to put in a little bit of effort, but not too much, to get the reward.
So I think about ways I get get back into those cycles quickly. To do that I try and cut up my work into the right size chunks, have a plan, and leave work with an "easy win" so I can get back into the loop again quickly.
Some examples. Hopefully I'm not descending too far into life-hacker wankery here ;-)
* I TDD code, and always try and leave myself with a failing test. If I'm interrupted and don't have a failing test I will deliberately break something or hit undo enough times to get a fail so I have an easy win when I get back.
* I don't track time on task. I do block out time for tasks - and track non-relevant interruptions during that time and see if there are ways to stop 'em happening.
* I run a personal kanban board for stuff so I can keep that continual ping of reward happening during the day.
* I breakdown tasks as they come in so they I can get little reward pings on a regular basis.
* When I have real problems getting into flow I give myself automatic rewards (do 20m on X then you can do 5m of fun on Y). Similar to http://en.wikipedia.org/wiki/Pomodoro_Technique. Once I get started the normal task-achievements often mean I don't actually end up doing Y - it's just a personal hack to get me started.
Basically - fake the reward cycle until the real one takes hold.
The reward technique never works with me. Because, to me, there is not 5m doing something fun. I cannot have something really fun in 5m. So I rather finish all the things in limit time and enjoy the rest of the day, it's the biggest motivation.
My only work solution to get back to the flow quickly is leaving something undone. Depend on which task, it will have different ways.
Btw, I really love the idea of leaving failing test :D
