This actually works whether or not your child has ASD. However, as parents it’s so easy to fall into only commenting on bad behavior. In order to give positive reinforcement, you have to be looking for the good behavior and good qualities that they are exhibiting and be quick to commend them. When giving commendation, be specific. Explain what they did that you liked and why you liked it. Did they do a good job making eye contact? Did they use the right language? Even a little thing can be a good thing to reinforce.
Another way to do this is to offer rewards when they have done something positive. Make sure that the reward fits the child otherwise it will not mean anything to them. The reward can be verbal or something tangible. The goal is to help them to make the connection that their good behavior equals positive reinforcement.
Each child is different, so different things work for different children. So be patient and focus on the positive. For more information on positive reinforcement, read Being Proactive in Therapy and Research.
If you are parenting with a spouse with Asperger Syndrome, download a free sample chapter from my newest project Parenting with a Spouse or Partner with Asperger Syndrome: Out of Sight, Out of Mind.