Two things do it for me...

1. Engage in a repetitive but not quite mindless activity.
   I used to work as a night auditor in a motel, and my best 'mindbreak'
   happened while Z-ing out the cash register, which required a long 
   sequence of keystrokes and ledger entries, always the same order
   but needing careful observation.  In more modern times, a data-entry
   or spreadsheet task will do it.  (Maybe a mantra or a rosary would
   do the same if you're so inclined!)

2. Lay back and deliberately relax the muscles around the scalp
   and neck.   This seems to change the circulation patterns in the 
   brain, and allows all sorts of latent memories to pop out and 
   connect into new forms.

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