If you go to www.hennapage.com they have a lot of great information about it! You can even request a CD-ROM with all the books on it for free! Other than that everything I've found has been from Googling "Henna".
I hope this helps!

I will start by saying, i am not an expert .. i just did some henna on both hands., so was just down this road..... first tip.. don't do both hands at the same sitting ..lol... for many reasons including after you are done, you won't be able to do anything while you let it sit...

i practised drawing with my left hand the shapes I wanted for my design first... I used a tube of fabric paint on a paper towel.. then my hand just because it washes off it turned out ok.. if you are doing an elaborate design.. try drawing it lightly with eyeliner or something first then it is just a matter of going over it...

since it is easier for me to draw what I see, i made sure i had my design printed out so i could look at it while putting it on myself...

good luck! :)