John,
Sorry to hear your deal fell through, but hopefully there will be other offers around the corner.
My two cents is that a strong earnest deposit -- say, at least 5% of purchase price -- speaks volumes and will prevent flakiness for just about any reason. And if the buyer does fail to follow through for whatever reason, that amount is likely worth your effort to have the deal fall through and start over.
You could always not allow a contingency for the buyer to get their earnest deposit refunded if they fail to obtain financing for any reason, or particularly for not being able to raise a stated down payment (say, 10-20%). Well qualified buyers likely won't mind not having that contingency available. Sure, you might have a slightly smaller pool of potential buyers, but most of those might not be buyers you want to deal with anyhow, as you just experienced. Best of luck.