We are in the category of people who didn't sell anything, in any downturn, ever, but I did rebalance (without changing my asset allocation). But some "background" information might be of use here.
My wife and I were always good "savers", but like many here, we were not good "investors" in the early days. Like many back then we were chasing the hottest mutual funds. We had twenty to thirty small mutual funds like Mutual Beacon, SafeCo, Brandywine, Janus, Berger, Oakmark, etc. Then, in the 1990s I became a Boglehead. I discovered "The Diehards" while reading Money Magazine while trying to discover the next hot fund. I kept reading articles about low cost index investing, on the Morningstar Diehards site, and elsewhere, and it made very good sense to me. I decided to move all our investments to Vanguard. It took a few years.
We had hurdles to overcome in moving our investmens. In the late 1990s I was having trouble selling the funds we owned because they were doing so well. Luckily we didn't have a whole lot in mutual funds at that time (we just had a lot of mutual funds). As an example, one fund I struggled to sell was Janus Mercury. It was a very hot tech fund and it was doing unbelievably well. How do you sell a great fund when it is doing so well? We had over $12,000 in that fund, and I decided divide the total by 12 and sell that much every month over the next year and move the money to Vanguard index funds. I followed through on my plan. I sold that amount every month and moved it to Vanguard. By the end of that one year period, I had moved the entire original amount out of Janus Mercury and into Vanguard. The trouble was, the balance in the Janus Mercury fund at the end of that year was then still over $10,000! I finally bit the bullet and sold it all, just before the tech bubble burst. Skill? Great market timing? No way. Just dumb luck, which came from reading all the stuff the Diehard's had to say. We also got out of Legg Mason Value Trust and almost all of our other funds before the market imploded. Of course the index funds went down too, but not nearly as much as the funds I had been in. Again, luck, and reading the Diehards posts. So, when the 2008/2009 crash occurred, we just held pat and within a year or two the market bounced back and we were amazed at how well we did. It was a simple strategy, but as the saying goes, "it was simple, but it wasn't easy".
If we had pulled out of the market on any downturns, we wouldn't be where we are today.
I retired in October of 2001, right in the middle of the tech bubble implosion, with a pension that met some, but not all, of our immediate needs. Not a good time to retire, market-wise, but we were almost fully indexed by then and we just did it. All has worked out well. We have never sold during market downturns. The only actions we took were to rebalance. We have now been retired for sixteen plus years and at this time I don't see any need to change our tactics.
As a side note, I was never a high earner and never made it to the $100,000 per year in salary (wife made maybe 25% of what I made). However, we did achieve Flagship status a few years after we retired, again thanks to Boglehead philosophy. During our working years I maintained an asset allocation of 80/20, but after we retired I throttled it way back to 75/25, and we are still at 75/25. I'm getting conservative in my old age.
I write all this to thank not only the Bogleheads, but also the Diehards. Ya'll deserve the credit! Stay the course.