Hi Becky,
That is a complicated question but a good one!
A lot depends on your child's age, personality, needs and how his/her ADHD affects him.
First, it's important to first make sure that the ADHD treatment the child is receiving is optimal. If that isn't in place, then working on improving behaviors will be extremely difficult.
In general, it's found that using positive reinforcement rather than punishment is a good way to help improve behaviors in children with ADHD. Catch your child "being good" and praise him. We become so focused on bad behaviors, that we often overlook the times they are acting appropriately.
Rewards can be time allowed on the computer or playing video games. Some parents use tokens that the child earns, then trading them in for prizes. With younger children, it's important to reward them quickly, as they typically cannot think far enough in advance to want to earn something if it takes weeks to do so. Children with ADHD are often impulsive and have a hard time waiting or thinking ahead. Immediate rewards are usually more successful.
Children with ADHD also thrive on structure. Keeping their routines the same each day/night will help keep them steady.
We have some very helpful articles here at ADHD Central on young children and ADHD that you might find helpful. You can find them HERE.
Hope this helps!
Terry