Do nothing! Sometimes you just have to do nothing! Especially the first 5-10 minutes of receiving a news or encountering something that disrupts your calm thinking. To me it happened few days ago before the markets started crashing. I had accumulated around $300 in USDT in one of my exchange accounts. I was eyeing to buy ETH at a discount for a long time, so when I saw the price of around $1,400 I almost impulse bought with all of my $300 with it. Then I just stopped and actually quit the app. Later I used those funds to buy ETH with couple of different limit orders the lowest of them being around $1,1k.