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You know you should buy low and sell high. Selling now is sort of the opposite. What I do is let it ride (my base allocation is 75/25 stocks/bonds plus roughly 1 year of expenses in cash) until the market is 20% down. At that point I change my AA to 80/20, adding to my equities (buying low). If the market continues down I again shift my AA by 5 percentage points for each 5% the market drops (from the original peak). At 40% down I'm all equities and thinking about how I can shift to more aggressive equities. If I run out of cash I sell just enough of my best performing (least down) equities on a monthly basis to meet expenses. That's where I was in the 2008 recession.
I'm a lot looser on the market recovery timing. Ideally I don't return my AA to normal until the market has fully recovered. But I usually hang on a little longer if the recovery looks like it will continue.
All of that, while not too hard, buys something like an extra 5% portfolio gain over just holding on to your base AA the whole -40% down and back to a full recovery. Not super significant in the long run, but it's something, and something to do. I can see where simply doing nothing has its appeal. For those who don't want to fuss with it, sticking to your normal AA and rebalancing if you can stomach it will work just fine.
Of course that all assumes that the market will behave something like it has in the past...