Time: Henna takes time! Allowing the mud 4-8 hours to soak into the skin will produce a darker stain. After 24 hours the skin can no longer absorb any more of the dye.
Avoid water: Water prevents the henna for developing fully. If you wash off henna you will have pale orange stains that do not fully darken. If you must get wet, apply a thin coat of vegetable or olive oil to the developing stain to protect it. Chlorinated water will kill a henna stain.
Warmth: Henna grows in hot climates. Staying warm helps the dye molecule feel at home and give you a darker stain. ( ok, maybe not feel at home, but it DOES effect the final stain!)
Air: Henna needs good old fashioned oxygen to complete the oxidation process. it is ok to wrap your design while you have the paste on, but once the paste is off, the new stain needs air to mature from orange to beautiful brown / orange.
Lemon Sugar Mix: Originally Henna paste was mixed by pulverizing fresh henna leaves and applying the mush the the skin. The henna would dry, flake and fall off very quickly. Lemon and sugar was added to the top of the henna to glue it to the skin. Now we know lemon and sugar do help henna, but it works better mixed into the henna and not applied over the top. Additionally, in Missouri and Kansas we have allot of biting and stinging bugs. Bugs that love fruit and sugar and that do not mind a nibble of human with their yummy treat.
Vicks Vaporub: Vicks contains camphor and eucalptious, both develop stains. However, Vicks is mostly petroleum jelly. Petroleum smoothers Henna and prevents the development of mature stains.
Sunblock: Sunblock is designed to provide a barrier on the skin. That barrier prevents the henna from getting the oxygen it needs to develop a rich stain.