For altcoins, buy low and sell high, and you will make a profit. Although this statement is correct, it does not go for all coins. Some people think that when the market rebounds, their altcoins will rise with it. But the result was leaving the market at a loss.
So choose wisely and follow up the project. See what their developers are doing, or just hiding behind the prevailing saying.
Picking altcoin must be ready for something like this. If not, just take it easy and pick big coins in market. But sometimes, coin which maybe have bigger chance to be dear or maybe not pumped again, usually have 1 or maybe 2 moments when it is give big profit for it's holder. But after that, it get dumped and developer leave their coins and who late to sell usually who mostly suffer because of it.