Although I have not been officially diagnosed ADHD I find that I am very distracted and have trouble gathering my thoughts. Here are a list of things that I have found help.
-Make lists of short attainable steps to get done what you want to do. The list will probably pop into your head in the wrong order, but once you have captured them all you can go back and organize them. At least this way you capture what needs to be done before you mind gets distracted and you forget what you were doing and what the steps were.
-If you are working on a computer shut down notification icons like email, social networking sites, and so forth. You are likely to lose focus and be distracted when a notification comes in or you will be tempted to go check sites for changes.
-Exercise. My dad often says that teachers told him that they could tell a big difference between when I walked to school and when I got a ride.
-Make it interesting. ADHDers are capable of hyper focus if it is something that interests them. Find ways to use that to your advantage to get lots done quickly. Find ways to make it interesting or even entertaining without creating more distractions for yourself.