All of previous posts are right, and each one contributes with one or another fundamental aspect of focus. Let me try to add this: first, have in mind the amount of time you can focus. Then, you can start building and getting stronger.
Take a day just for training your attention: load a lot of micro sit and gos, for example, maybe those of $0.25 with 45 players at
pokerstars, or maybe a lot of
freerolls (because you would not lose anything), and play with maximum attention and try to keep it. Also, when tournaments goes for break in the end of each hour, you also need to stand up and walk away from the computer; don't be sitting for long hours, enjoy the breaks, maybe change the room you are at in your house, and then come back with your focus renewed.
Register the amount of time you could focus with ease. And try to grow this number. If you can't focus for 55 minutes straight (because the point is to ewnew the focus at the break), you should load some 9-players SnGs on PS (they last 45 min) and you could train your focus.
Also, avoid distractions: people, other sites, cellphone, etc. Many people say to keep the mind clean, but you need to keep your pc clean too.