I helped teach a couple kids who had ADD/ADHD as well as another kid who had similar problems, though for a different medical reason. All of them seemed to gain better attention over time (parents even said kids did better in school).
My only suggestion is patience. It is a slow and sometimes painful process, but the reward can be great. If their attention started to drift while I walk talking or demonstrating, I would call their name and then say "Look at me." or "Watch what I am doing." This seemed to allow them to regain their focus.
I also tried to have them verbally acknowledge me when I was done or ask them "What did I say/do?" and it usually allowed me the chance to help them if they didn't quite get what it was I was saying or showing.
Other times, depending on what we were doing, if they were truly spacing out and wanted nothing to do with class, I was ask if they wanted to sit out. Sometimes they did want to and other times it seemed to make them realize they needed to pay attention to participate.
Best of luck to you. In the end, you will just have to figure out what works with this individual person. Nothing works for everyone.