LISTEN... I can be terrible at this one sometimes because I have the attention span of a 3 year old, but it is so nice to actually be listened to! Makes me feel very lucky.
Look in the eyes often. Watch your friends sometimes. You'd be amazed how many couples can go a whole day talking without looking in each other's eyes.
Be appreciative and never take the other person for granted. Be supportive of their endeavors.
Like Greco said, TOUCH. You cannot underestimate the importance of it. For those of us not yet to the point of being together all the time, it's the thing you miss the most. Even better than sex (if there's any such thing
)