Ancient aztecs believed that you could "summon" or "call on him" by means of blood sacrifices. They used to take the tip of the Maguey leaf (http://www.dgcs.unam.mx/boletin/bdboletin/multimedia/WAV110507/269.jpg) and would perforate either their tongue, their ears or, in cases of dire need: their genitalia.
This blood was left to fall free to the ground while they prayed. I do not know (as I have never ever done it) if it works. I can tell you that the Maguey leaf tip is extremely hard and perforates flesh easily. the wound bleeds profusely and it stings A LOT. (I fell onto once once, and I got several dotted scars on my arm) BTW it takes up to three weeks to heal.