I use Lush Henna. I dye it once a month and my roots never show, I and don't bother trying to isolate my roots each time. It's way too messy to fuss with that! Since henna is basically a stain, you're not going to really be able to lighten your hair, but you can definately get it darker by doing the mix with strong coffee instead of water, or using indigo. (Or, if you're going the lush route, adding a chunk of noir or marron to the mix if your using rouge).
Also I have a step-by-step how to on my blog if you're interested!http://the-kitschen.blogspot.com/2006/04/how-to-henna-your-hair.html