Set alerts on your stocks so when they go below this set price, you buy more (while it’s cheap)
Set alerts on your stocks so when those stocks go above this set price, you sell those shares (if you are a trader, I’m not. I am an investor for the life. But if you see something that makes you think this company can’t go up anymore and that it may shut down or something then I guess you can sell.)
Note: I only buy companies that I think that I can hold for the next 20+ years.
If you don’t know a company that you are excited about, then buy one of the S&P500 funds
(you should grow with the market)