Buy and sell totally depends on you but i always try to buy in deep and sell it high though we all do same mistakes in everytime but for selling your coins you can follow markets conditions, fundamental news and technical history too.
^ I rather choose the keep buying than selling.
Kowing the fact that when you sell is a very simple way, from the start of your investment in BTC, you should write it down in what price in the market you have purchased your BTC and then, just aim an amount that you want to sell your BTC, in that strategy, it is impossible you did not know when is the perfect time of selling. I also suggest, don't be too much greedy in making a profit in crypto, because sometimes it will cause your losses.