I have a very very good friend who is amazing at doing just that. They're usually as far ahead in all their classes as they can be, all their homework gets done usually the day it's assigned, and therefore they don't have to stress about whether they'll have time to fit it in right before it's due. Definitely something that I need to work on.
So how does one focus when one is really tired from work, or really distracted because they're not working? Caffeine and food is the simple answer. Caffeine brings plenty of immediate energy, and eating healthy-good carbs and proteins-gives you more long term energy. But the real answer to it, as is the answer to most problem...well all problems, is prayer.
Here's another example, one that perhaps you can look at less personally than say work and busy schedules: Imagine a man were to make a vow. Imagine the vow was to live in responsibility, purity, sobriety, honesty, and forgiveness. Then, imagine that he shaves his head every day until his vow becomes his lifestyle in order to consecrate his vow. That man would be a bald man the rest of his life unless he discovered that his vow must be, in truth, a seven-fold vow, adding on prayer and that which prayer requires-namely humility.
Let's imagine another scenario: you are late for work, your car dies, your best friend's dog dies, a family member is terribly sick in the hospital, your beloved leaves you, the banks call all your loans, and you haven't eaten all day. What can you do? You can pray. And when you pray, humbling yourself before God, you will be offered peace, which you can ignore, and hold on to your stresses, or accept. You will not be given a "this is why..." answer. But if you are truly praying with a humble heart (and with a day like that you'd better be humble), you can know that God is in control. Your focus will be on Him, and when you keep your eyes on Him, whether when tired or at the end of your rope, you can walk on water. This is my reality.