That's absolutely gorgeous! :] Do you put anything extra in your henna? Was wondering because I do henna as well and boy would it be nice to figure out some secret to making darker henna lol I know Tamarind is supposed to darken it or make it redder... hm?
Once it's on your skin? Not that I know of. Though lemon juice does help a liiiitle bit if used while the henna is still on your skin and immediately after removing it. To make it last better, wash that area as little as possible and don't use soap there if you can stand it. Don't exfoliate the area in any way. And make sure to keep it well moisturized using some kind of natural moisturizers like aloe or cocoa butter... those extend-your-tan after-sun moisturizers works great, as do the body butter tubs from the body shop.
Best way to get dark henna is to use a a good henna mixture. The stain will depend on both the henna used and your own skin type.
Yay for henna!