Help with lightening natural blonde hair!

Hi all,

So my hair is naturally about an 8 or so, but I would like to get it up to a 10. I used to go get highlights every 3 or 4 months to lighten it, but I'm looking to avoid the salon for $ reasons, plus its more convenient to do it myself. I have been doing my own hair since I was about 14, and have gone from black, to blonde, to everything in between. Lightening is a challenge for me though because I ALWAYS go brassy. I don't know what it is, even ash toners make the natural hair brassy and yellow. Recently I tried the Ion Highlift Cool Blonde toner, as the girl at Sally's said it should lighten my natural hair a bit, and tone all of the yellow out. I mixed a container with that with the Ion sensitive scalp 20 developer and left it on for about 20 mins. My results were "meh". My bleached highlights were almost purple (which I can deal with), but my roots and higher part of my hair was a bit of a brassy/yellow color. I went back to Sally's to ask, and they recommended the Walla 7 to cancel out the brassiness. That did nothing - if not just made it worse. Its gotten better over the past couple of weeks, but I was wondering what I could do differently in the future?

Thanks so much!