I was a slave to colouring my hair and having highlights too a few years ago. My hair is naturally light brown and I used to use a home colour kit regularly (as soon as the roots started to show). I then started having professional highlights but found the same problem with roots showing as my hair grew. I was absolutely fed up with having to constantly colour my hair and wished I'd never started.
A friend of mine, who is a hair stylist, suggested that I try a product call 'Sun In' I don't know if you have this in the US? It's a very low concentrate of peroxide in a spray. Basically, when my roots started to regrow, I sprayed random strands with the spray avoiding the ends and older growth (probably once every few weeks to begin with - it just took a few minutes before blow drying, no waiting or washing out). This gently lightened some of the new growth so the line wasn't so harsh. I kept this up for probably about a year (it is such a painless process, I can't really remember). No-one seemed to notice that my colour was darkening (maybe they were too polite to say!!) It certainly didn't look harsh and unattractive though.
Anyway, I haven't coloured my hair now for five years, but I still use Sun In about once a month. I don't spray it all over my hair, but after washing, I select random strands and spray. The result is medium blonde and very natural (my mother actually said how nice my hair is now its back to my natural colour - although its actually about three shades lighter - she's obviously forgotten what my real colour is!!). Because I use the spray so infrequently it hasn't affected the condition of my hair either.
I must sound like an advert for Sun In!! but it really has taken a lot of pressure out of my life. I'm still blonde - but don't have the horrible re-growth lines or the bad condition of constant colouring.