Blonde hair turns dull?

I have blonde hair that I get highlited, and right after I get it done it always looks wonderful, but then it starts getting dull and kinda brassy and dark looking. I have well water but we have a water softener. I wash my hair 2-3 times a week. I use tigi dumb blonde shampoo and conditioner, and once or twice a week I use john frieda tone renew shampoo and conditioner. Just wondered if anyone had any suggestions to keep my blonde hair looking like it does when I get it done! Thanks!