Previously on nanach.net there is an explanation from Rabbainu and the Sulam where many "miracle workers" draw their power from: http://www.nanach.net/2010/03/see-yourself-see-god-sing-na-nach.html. Amazingly this week I came across a piece from the Noam Elimelech on this weeks parsha which supplements this concept beautifully.
The Noam Elimelech explains what Pharaoh says to Moshe and Aron, "make a wonder for you" which is strange because Moshe and Aron would be making the wonder to make a point to the Egyptians not for themselves (most people just read this verse as "make yourselves a wonder", but it still hints this meaning, that the wonder is to be made for them). The Noam Elimelech says that real miracles are completely new in the eyes of their makers, no matter how many times they've done them, if the miracle is true it will be a complete novel experience. Pharaoh was used to seeing magic and understood that even "holy men" could pull of tricks, so he demanded Moshe and Aron to demonstrate a true miraculous act of God - which would be a wonder even to them, to Moshe and Aron.
This is exactly the concept explained previously on nanach.net, that the MSS - the charlatan leaders often pull of feats, but they pull them off from the dead power of their stagnating in "old age" of devotion.