Some things every parent can do for their child is make sure the get plenty of sleep (8.5 - 9.25 hours a night) and give them Omega-3 (fish oil) supplements. (Make sure they are molecularly distilled to get out all the mercury.)
Some children with sleep deprivation are misdiagnosed with ADD and Omega-3 supplementation has been shown to help some ADD sufferers.
I'm not familiar with the Omega-3 supplements (I don't discount it; I'm just not familiar with it as a treatment), but I agree that a good night's sleep can be beneficial for ADHD as well as many other things.
To add to what's been mentioned previously, keep the kiddo away from anything with red dye, large amounts of sugar, and especially minimize or eliminate their caffein if they're on medication.
However, if they're not on medication, caffein may actually help them by slowing them down some. I talked to someone who said his daughter drank a lot of coffee because it soothed her. This was before ADD/ADHD was a diagnosis.