Tante - I don't do much day trading, but when I do I try to follow the advice of Warren Buffet, which is (paraphrasing) "Be greedy when others are fearful, and be fearful when others are greedy." There are key words on the forums that you can look for which make it really obvious how people are feeling. For example, "crash," "panic," "sell-off," "trash," "dump," ...etc. All of that is on the bitcoin buzzfeed right now, indicating it's a good time to buy in. I don't do much selling, because my goal is to buy in cheap but then stay in for the long term, but there are similar words letting you know when to sell: "soaring," "gold rush," "huge investment," etc.
What I've learned the hard way is that I need to decide when to buy at the beginning of the price drop - otherwise I'll miss the bottom and get screwed. Rather than try to predict low it will go, my system is to let it drop as low as it wants to go, then buy as soon as it rises more then about $10. How much I buy depends on how far it drops. For example, btc is at about $450 right now. I have no idea how low it will go, but you can bet that it will rebound almost immediately after it bottoms out. So suppose it drops to $400, then starts going back up - I'll declare the crash over and buy in when the price hits $410/$420. If it drops all the way to $350, I'd buy in when it went up to around $365, but in that case I'd buy more.
You could play a similar game with selling, but personally I think that btc and drk are going to continue rising (on average) for a long time, so I'll be hanging on to them. By the way, I should emphasize that I'm a physicist, not a financial adviser - I bring all this up because it's interesting, not as advice!
P.S. PBOC = People's Bank of China - the jokers who like to "ban" bitcoin every other week, presumably to give their friends a chance to buy in cheap.