This is something I ask myself from time to time. It's hard, because I never know what's coming. But mostly I just shop low and hold. If I know it's a quality company and I see potential in it, I hold and don't sell. After all, for me, long-term investing is the best method and it's really not about whether or not I take profits and I don't sell when the stock is down 20% and vice versa. Of course, there are some stocks where I would probably take that profit.

I buy individual stocks based on some research and an investment thesis. If the stock fulfills that thesis or exceeds my expectations, then I do additional research on what to do. If I then don't see the company making sense relative to, say, large index ETFs (within their average historical performance), then I realize gains or allocate them to other companies (I'm thinking of tax optimization, of course). For example, I recently realized about a 15% gain on Walmart this way, as I continue to see no long-term value in it. I also sold off Iron Mountain, where I took a profit of over 25%, as I no longer saw much further upside given current valuation and other metrics. Not to mention stocks that fly out quietly by 100% and then their PE ratio is in the stars (NVIDIA, META, etc).

This question weighs on me too, then the stock will drop x% in a few days and I could jump back in with a bigger chunk of money. So I decided that companies where I really see some possible future growth I just hold and when they are on ATH I don't buy and rather look at other stocks, currently dividend stocks. Dividends come in every year and you don't have to worry about anything, I reinvest the divi that comes in.