i know you are asking about henna. but i've been enjoying regularly conditioning my hair (every two months) with neutral henna. it's really the cassia obovata plant, which is closely related to the henna plant, only it doesn't impart a stain.
after preparing it, i wrap my hair in a plastic bag, then wrap a towel around it and leave it on my hair for 1 to 1.5 hours.
after rinsing it well and following with my rinse-out conditioner, my hair is left remarkably shiny, well conditioned, and moisturized.
i "henna" every couple of months at the time that i trim my ends. tomorrow is actually my next henna treatment.
i use the rainbow brand which i get from whole foods.
"Dogs stink too, but I like dog stink." ~ rileyb