As, I mentioned in my profile, I sometimes have extra difficulty with my focus because of my ADHD. What’s helped me is choosing a broad area of focus that includes a lot of collaboration with other fields and disciplines. That means that I’m always learning, and that keeps my interest fired.
If you choose an area of science (or really anything) for reasons other than blazing curiosity and interest, like money or prestige, somewhere along the line, you’re going to have trouble keeping focused. I love what I do and am always looking for new ways to share that interest because I can’t contain all the excitement inside me. If you feel this way about your area of study, all the long hours and sacrifices will always be worth it, and you won’t find focusing that hard at all!
I was not always focused in school. If I didn’t like a class I would read under the desk, or pass notes or day dream. I was genuinely interested in my science classes, so I paid attention so I could learn more. I was lucky to have a number of fantastic teachers who made the lessons interesting and interactive.
If you have a subject that you find really easy to focus in, and that you enjoy, find a way to get a job in that subject. Lindsay is absolutely right. I know a lot of biochemists who went into science because their parents pressured them into it, or they thought they could make a lot of money and they are miserable. If you love it, its easy to keep focused through difficult or boring work.
When I was in school I studied music and literature, so things were pretty different. But what kept me going was knowing that at one point you can become THE expert in the world on something, or know exactly who those experts are personally.
Honestly, in high school and college I think it was because I had a balanced life: ate healthy foods, spent lots of time with friends and family, and worked out regularly. I let that slip sometimes in grad school because I was so busy, and my body/mind would really start to feel terrible.