How do you get darker henna stains?

Get the best henna you can find. 
Ask folks on Henna Page Forum what they're using lately that is giving great stain!

(The henna above was done with  2002 season crop pure henna from Yemen, "terped" with Tea Tree EO, Clove Bud EO and Cardamon EO, lightly sealed with New Skin, heated and steamed, left on overnight, picture taken 36 hours after application)