Well, the article "40 simple ways to show respect to parents" answers your question in a detailed manner. If you have not read the article check it on my profile page articles listing. Or you are asking the question because you feel it difficult to follow keep in mind that you have to respect them at least the way you do to your boss or professor or maybe that cool uncle around neighbors, only with a little patience and much love. To add to it, I say respect them the way you would want your children to respect you. Follow the tips in the article and more from what your heart says and keep doing it until you make it a habit.
I know it could be awkward just to start doing things like kissing their forehead when you have been away for a long time. But, it will feel natural if you keep doing it for some time.