I think the reason the challenge has not been beaten yet is because of the sheep-goat effect. If so then this effect needs to be overcome in order to beat the challenge. I have an idea how to do that.
First, I'll explain the sheep-goat effect a bit. Sheep are 'believers', and goats are 'non-believers'. A psychic challenger takes the million dollar challenge and fails. Why? Because the unconscious psi of Randi and all his goat fans interferes with the psi of the challenger, and no one is the wiser. The sheep-goat effect takes place under the threshold of conscious awareness.
So, here's what we need to do.
There is a correlation between meditation and psychic strength. I think the unconscious psi of the Randi goats could be overcome by a large group of experienced meditators working in psychic unison on a specific task. The more the better, but I think about a hundred or so should do it.
So step one would be getting the meditators together so they can practice group meditation. Step two would be arranging for a group application to the challenge. That could be the hardest part.
Step three would be arranging for the challenge to take place at a time and location that is calculated to be psi-conducive. Then get the meditators together and give them time to get in sync and presto.
Oh, and don't forget a camera to capture the look on Randi's face.