Back when I was a blonde, I developed a bad case of blonderexia. That's right. I was always trying to go blonder and blonder. I couldn't get enough blonde! (The cure ended up being a dramatic switch to my natural brown, as I outlined here, but that's another story.) Anyway, what I'm trying to say is that I understand the disorder quite well, and if you happen to have it, then this is a line you might want to look into.
The problem with blonde is that in between colourings, it can get a little dull. Add that to the perpetual root re-growth issue, and you've got a problem on your hands. (I don't care what they say about roots being a new trend. Unless you're a glammed-up celebrity, it just looks messy and unkempt.)
But back to blonding. This line is genius. It contains lactic acid and chamomile, both of which gradually lighten hair by breaking down colour pigments in the hair shaft. Kinda the way lemon juice lightens your hair if you spray it on before going out in the sun. But not, of course, drying like lemon juice—if I recall some teenage beauty experiments of mine, that trick didn't always turn out so well. (Think blonde straw.) So this can satisfy your blonderexia, as it were, without the drying effects of some, er, unconventional methods you may have tried.
If brassiness if more your problem, however, then there's another option for you from John Frieda: Colour Renew Tone-Restoring Shampoo and Conditioner. It contains lavender (oil? extracts? I'm not sure) and something called "optical brightening technology" to cancel out the yellow tones.
If I were blonde, I'd hedge my bets and use both (on alternate days) to keep the brassies and dulls away for good.
Priced at $12.99 each. Available at grocery, drug and other mass-market retailers across Canada.