I would echo what has been said earlier about finding somehwere quiet and visualising the lesson on the ground. Practice does make perfect and I think one owes it to ones student to have rehearsed the lesson before giving it.
In another life, I refereed rugby. Before each game, I would take ten minutes to visualise the kick off and the first sequence of contacts. It worked to eliminate the brain fade and especially in a big game, by having rehearsed the first couple of minutes I was ahead of the game and ready for anything.
Flying is no different and I still do the visualisation exercises such as the EFATO routine before I even get in the plane.