Simple actually I sell when the price is high and I buy when the price is cheap
So I get the profit from the difference between the sale and purchase
This "strategy" is really simple but i think everyone use it, as this is the easiest way to get profit from trading, you won't lose money if you sell for more than you bought for, problem is that sometimes you will have to wait days or even weeks to sell.
Personally i sell only when i am sure that i earn from it, it would be dumb to sell when you lose money, unless you know that this currency will crash or so.
It`s simple when we talk about it, trying to sell when price is on top and to buy on bottom is like mission impossible for most of us. I also sell when I`m in profit, many times I sold much before the top but that is because I`m not greedy to wait too much and lose what I already have.
When to sell is individual decision, that depends most of the fact how much is someone willing to risk. I`m not a risky trader, and I`m not long term holder, so when I see double price I`m preparing for selling.