Before applying henna many people rub either eucalyptus or clove oil onto the area being tattooed. This is said to help the color set in the skin. This is a crucial step. Don't pick up stuff, this might smear or ruin the design. Most people let the henna dry over-night.
Some people apply sugar water onto the henna to keep it from drying too fast and flaking off.
If you want the color to last longer, scrape off the dried henna and put some oil onto the skin. Many people suggest that the tattoo should not touch water for up to 24 hours. This is just one method used by henna artists. Henna naturally produces a color that ranges from a light orange to deep maroon or brown color depending on the quality of the henna, but never black or any other color. These colored henna products most likely contain chemicals to produce the color they are advertising.