Keeping you eyes open and be aware of market movement.
Otherwise you will be holding them for a long time won't be selling.
And also try not to be impatient. Some people are panic selling when the price goes up a little, they think it will go down again but most of the times it doesn't. Better just hold your money until most people in Speculation say to sell your bitcoins.

Obviously. Some people sell their coins too early because of panic, they think that their profit will turn into loss if they don't sell them now because of the price drop, and some people don't sell their coins on time by thinking that they will earn more profit after selling them later but later the price drops and he pick ups a loss from that.
Both of these people will be in loss, first one because he could earn a little more profit, and second one because he could earn enough profit.