Any more tips about highlighting?

I am not sure about how to touch-up my highlights. My hair starts out a little darker from the roots, and gradually gets lighter towards the ends. How could I get a more even look? Should I not apply the lightening mixture to the ends at all when doing a touch-up? (I have been using Loreal's frost and tip with a cap). Does there come a time, after years of highlighting, that you need to alternate methods of coloring hair? Should I try an all-over color, and then add highlights to that?