There is an old tradition in New Orleans, if you ever make your way to the St. Louis Cemetery #1.
There is a tomb in front of which one often sees offerings of flowers and candles, and the structure is covered over with X shapes scratched or drawn on the walls. This is the tomb of the famous Voodoo Queen Marie Laveau.
History -- written largely by whites who didn't approve of her practice -- has claimed she began as a hairdresser who learned secrets about her wealthy clients, and passed herself to them as a fortune teller and magician. Recent scholarship finds no evidence of her having been a hairdresser, and instead it appears she married, and was subsequently widowed, at a young age. She then took up with a man descended from French nobility. She was apparently left fairly well off by her gentlemen, and either for fun or as public service began pitching her Afro-Catholic Voodoo practices to the public. Her public Voodoo Dances became a tourist destination, and to this day shops in the city promote themselves by using her famous name.
Voodoo believes in not just life after death, but also that the spirits of the dead will be able to assist you in your worldly affairs. Marie Laveau became a popular spirit to request for help. The tradition is that you draw three Xs on her tomb and make your wish, then leave an offering for her -- however, the owners of the tomb now discourage the markings being made. It is recommended instead to just leave the offering with the wish. One man who claimed to be a descendant of Laveau recommended that one go to her tomb and promise a certain amount of money be paid upon her granting of the desired outcome; the money could be donated to a Catholic church.
Actual money is a traditional gift to leave on graves in hoodoo rites, but for a popular tomb like Laveau's, you would probably not want to do this since cash likely to be seen and taken by other visitors; perfume and alcohol are other traditional presents, and can be poured out in front of the tomb or splashed onto it.
Marie Laveau never worked cheap when she was alive, and her services are probably even more in demand now that she's dead; so be sure leave her a nice, substantial gift if ever you want her services.