Re: mehndi
Exfoliate. As skill cells come off, so does mehndi.
Do a lot of household cleaning (with the appropriate chemicals, of course).'
Putting whitening toothpaste on it can help.
If you're able to purchase Aveda products, get their exfoliant. It's non abrasive, and to demonstrate how effective it was to a class I doused a pad in it and rubbed it on my arm that had mehndi all over it. My mehndi was dark, and the exfoliant nearly made it disappear--without pain or side effects.
Otherwise, just wash your hands a lot and wait :)