I'm not ADHD, but I have always found it hard to keep my mind centered on relaxing, but I found a pretty good method that keeps me focused when meditating for rituals or even astral projection (I found it in a ritual for astral projection even).
When you're trying to center yourself, count your breaths, visualize the numbers in your head. I've found by about 20, I can't think of other things and you start over when ever you become distracted.
Also, music helps to drown out any noise from the outside world (cars, TV, etc.) that can be distracting and I used to blind fold myself to keep out light and to keep from being tempted from opening my eyes and looking around. It forced me to relax inside my own head and now meditation is a lot easier.