It's good to read threads like this. It helps to see solutions that others have come up with for this problem.
I depend almost completely on Google to bring in my users and advertisers, and it's not cost effective any other way. My solution is to sock away as much as I can during the good times so that I can survive during bad times, if necessary.
I figure during any given year, there's probably a one-in-five chance that Google will erroneously penalize my site, and penalties generally last a maximum of 9 months, so I should be stashing away about 15% of my revenue every month.
It's not a sexy plan, but it helps me sleep at night.