In addition to "Remember where you came from," I'd also add: track regular, incremental progress. Entrepreneurs have a tendency to move from big win to big win, while forgetting all of the little steps that got them to that point.
Keeping track of daily or weekly progress (even as simple as iDoneThis), can help minimize the amplitude of the downswing. It will still be there. It's inevitable. But this can help take some of the sting out of the low points.