Ex. If you're 40 and think you'll live to 100, try to hold 40% bonds and 60% stocks.
Those things shouldn't change in response to short-term market fluctuations, but not all external events are limited to short-term market fluctuations.
The key is to maintain a mix that allows for short to medium term fluctuations in the market (and economy) like this.
EDIT: I highly recommend the book "A Random Walk Down Wall Street" for more details on asset allocation strategies at various ages.
Why are stocks a valid strategy (where there's possibility of easily losing 20-30%), while losing a few percent due to inflation in something like a savings account isn't?
also see cFIREsim for an idea of the math: you're basically guaranteed to run out of money if you try to park in safe havens and you end up living a fair bit after you retire.
If you are struggling to stay within a "safe" withdrawal rate of 2-3%, you may find that equities are your only chance of getting the returns that you need.