I get my hair professionally highlighted and lowlighted about 3x a year. My big secret for extending the color? I wait 7 days after the color is applied to wash my hair. Then, I only wash in very lukewarm water and I avoid hot styling for the first month.
My color usually lasts 4 months instead of 2 and it looks very fresh.
Of course that's just me, but the longer you can go before washing after color, the better for your extended color vibrancy and minimal fading.