Been a while now. I look at what successful people do with this problem aand the answer is meditation. In fact, aabout half of the answers given boils down to meditation - light naps, sitting in the toilet, light exercise, tea. It's more detailed than Dave Newton's answer so I'll answer myself.
The idea behind meditation is to force yourself to focus on something of low mental activity. Breathing exercises are common because it's a way to force focus. Similarly, you can stare at a fire. In modern times, it could be a treadmill, a repititive game like Farmville, or humming along to lyrics on a headphone.
This puts your mind in a low activity state where it can process/store recent info and clear junk resulting from overthinking. It's similar to sleep, except that it can take place within a few minutes and you don't have to worry about the shock of being waken up.
You can't be too relaxed or too focused. Ideally, it's just sitting down but not laying back.
At work, this kind of concentration looks like daydreaming or zoning out, sso thee best alternative is probably sitting on the toilet if you have a work culture with no privacy.