Consistency. Consistency is a good way to show her that you'll always be there - gives her a sense of security. This doesn't mean knock all spontaneous-ness out the window, but have somethings you're consistent with.
One thing I would be careful of is making sure you're not being overlooked. Although we may like to believe that all people are good people, some people will think "you'll always be there" and use that as a way to mistreat you or not give you what you truly deserve. A relationship shouldn't be about people "using" each other.
