One GF seemed to want to hear it all the time from me, and that took some of the meaning/magic away, in my opinion.
I agree with Missy that it should just as much be in the things NOT said, or the things done instead of said.
Although I will grant you that perhaps over the telephone or via just email, where you don't have visual cues, maybe the little reminders become a little more important.
I try to keep it through things said and only say it when I truly feel it...and no I don't think it's necessarily a commitment thing, as even my closest friends know that I love them, in a different way.