I have tried henna in the past and I used it, because yes I am vain and I want big thick long hair, so thanks to Curly Nikki. She is available on http://curlynikki.com/. I used her method.
The henna, I got from my local Asian supermarket in Shepherd's bush Market. So with one packet of henna, I mixed warm nettle tea, 2 tablespoons of honey, one packet of toners and shaders, hot cinammon, 2 tablespoons of extra virgin olive oil. The henna was mixed until it was in a smooth paste, then I covered the henna and placed in the airing cupboard over night.
I conditioned washed my hair, then applied the henna for four and a half hours, washed and rinsed thoroughly with conditioner, then deep conditioned for 30-60 minutes, then rinsed with cider vinegar rinse.
Very pleased with how my hair came out. Curly Nikki, blog is very informative and she has lovely hair.