I have been dying my hair darker for about 20 years. I am a very very light brown/dark blonde, and dye it medium brown. I used to do it every 4-6 weeks to freshen up the colour and do the roots.
Since before Xmas I started using aveda's black Malva conditioner and I haven't had to dye my hair since.
It's pricey (£16) but have used it every day or every other day for about 5 months and there's still some left. (I don't have long hair). I reckon that with not spending on hair dye, it's probably cheaper.
I can't believe nobody ever mentioned this. I've been moaning for years to various hairdressers about my dye fading and they've never mentioned there are colour enhancing products that actually work!