1)Make sure you wash your hands with soap before you have mehndi applied to your hands to make sure your hands are free of any oils or creams.
2)Keep the mehndi for a longer period of time. 6-7 hours is what is usually recommended but any longer is even better.
3)Many girls use the smoke from burning cloves to darken their mehndi. Just cup your hands and put them at a safe distance over the hot cloves.
4)Use the tried and tested lemon and sugar mix. Dab a mixture of sugar granules and lemon on your hands once the mehndi has dried completely. Alternately you can also make a sugar syrup and then add a few lemon drops and use this mixture.
5) A different way of giving enough time to the mehndi is to make sure you have made your last trip to the washroom for the day and then get the mehndi and then hit the sack once it has dried up completely. Wake up once in about 3 or 4 hours and rub your hands and take off the mehndi. Apply vicks on your hands liberally and go back to sleep with your fists closed.