One thing you could try is the Pomodoro Technique (google it and you will find all the details.)

Another is a little exercise when the inattentiveness is worst.

It might help to work on figuring out whether environmental distractions play into it. For example, working in a carrel in a library where there are no interesting things around you might be easier. You might do better with complete silence, or with music in the background. You might have an easier time with bright light, or low light. You might need to turn the Internet "off" by shutting off wifi or pulling out the ethernet cord for awhile so that you can't screw around online. There is also computer software to help you enforce limits on being online, doing email, stuff like that, and others that provide cues to "get back to work" (these are much too distracting to me.) These are typical learning style details that become much more important if you're distractable. I usually work in complete silence and get very cranky at any noise but sometimes headphones are OK. I work at home where I can completely control the environment around me and there are no other people moving around in my peripheral vision, which distracts me a lot. I usually work in low light because my computer monitor is HUGE and the low light makes everything else "fade away" into the background.

Otherwise, well, the ADD folks are not necessarily the best people at figuring out how to deal with being distracted. Most of us can't overcome it without meds - I can try all I want but unless I take my meds, I'm getting up out of my seat every 10 minutes.