I have adhd, and I find that when I have to sit down and do some work, I need to reward myself regularly, and set myself small goals. For example, I may decide that for every 100 words I write, I can get up and go for a walk around my garden. This helps clear my mind, and allows me to get my work done.
I also get more work done when I am in an environment with few distractions, e.g. a library or empty classroom.
I'm not sure if your question is about adults with adhd, or children. I'm speaking from an adult's perspective. However, I think the same techniques I've described may also be useful to use with children with adhd.