Easiest way is just time. Most light stains will fade by morning, but if you have a deep, dark stain (you left skin fully exposed and immersed in henna) well, you can only really speed up the fade time by applying nature oils (coconut, olive). They may help fade the stain faster and along with some direct exposure to UV(sun) light. Best advice: wash up very well after finishing hair application. If it stays on the skin for too long, it's simply going to stain (for a few days in worst case[full exposure & left on skin for 2 hours]).
*The Pure Henna will stain longer (up to 2 weeks). Just wash up well to avoid.