buying low and selling high is the thing to do. but I see if we make purchases and sales at high prices maybe we should also wait for prices to go back down to buy back, because if we buy at high prices it is a speculation and if prices can be high it can be our profit but if prices are at low position, it might be a loss for us.
It's all about being apt in our decision making, you can be conversant with all the strategies in trading and have all the experience needed, but if you make decisions without prior proper thought then you cannot be said to be a good trader.
As a trader or investor, you need to know the best time to buy,sell,cut losses etc.
I personally sell immediately my target is attained for that particular investment, then i wait for a favorable time when the price would be lower than the price I bought previously to buy again, you just need to make good decisions all the time.