Okay, I'll respond to this zombie thread, too!

I henna about every six - eight weeks. I've started heavy oiling right after I am done with my treatment, and I keep it heavily oiled for a day or two. Also, I only use water to mix in with the henna. Perfect results, every time.

As far as build-up goes ... I'm getting a lot of color build-up and am starting to fear going too purple-auburn. I just hennaed two days ago, so I'll wash my hair tomorrow maybe and post a pic to see what it's looking like these days.