One of the greatest ways to show our parents that we love them is by asking them for advise. This shows them that we respect their thoughts. It also shows them that we think that they have made wise choices and have wisdom to share.
Also, a simple "thank you" is a great gesture. Parents sacrifice a great deal for their children while they are growing up. We can thank them for providing home cooked meals, placing bandages on our knees, and staying up through the night when we were sick.
And possibly the most important way we can express love to our parents is by simply spending time with them. As we get older we get busy with our own lives but forget about the people who truly matter.