I would like to pour concrete over an asphalt drive, the asphalt is beyond repair.
Is this possible?
Do I need to prepare the asphalt in any way?

You can, but depending on the state of the asphalt or the construction of the new drive it may crack and spall with time and use. If you are replacing the drive because the surface has simply deteriorated away under wear from your car then is shouldn't be a problem to overpour as below the asphalt the ground base is essentially sound. However, if the surface has large dents or hexagonal cracking than the ground base below is not suitable and you should do something a to deal with this.

Firstly concrete has very little flexual strength because it is weak in tension. It also undergoes shrinkage due to temperature and curing. Both can lead to cracks in the face of the concrete that will reduce the service life of you drive.

So if you think the ground below is fine than use a 100mm (4") thick concrete slab, with joints at no greater than 4m (13ft) centres. If you think the ground below is poor than consider either digging out the asphalt and soil below and replacing with some well compacted stone 200mm (8") and then the slab; or reinforce the slab with steel mesh in wich case you can have fewer movement joints say 6m centres (20ft).

If you need any more help feel free to ask.

I wouldn't recommend pouring a new concrete slab on top of damaged asphalt. A lot depends on the type of soil base you have in your area and the cause of the asphalt failure may also cause the overlay of concrete to fail for the same reason.

If it were me... I would start fresh by removing the asphalt and insuring a solid, non-expansive base for the new slab.

I agree with both the previous answers. You can pour concrete over anything. It is your sub base layer that will determine the life of your concrete. COMPACTION, COMPACTION, COMPACTION. Settlement is the reason for potholes, stress related cracks, and un-even surface conditions. COMPACT the sub surface with a plate compactor the more the better. Use water during compaction. Water is the mortar during compaction. If you use dirt for a base, 'not recommended' but for illistrative purposes. Place water on ground prior to placing the dirt used for the sub base. Compact the dirt over the water and you will have as good a sub base as dirt can be. Remove soft spots, they increase the probability of stress related cracks. Army Corps of Engineers determined that a crusher run gravel is best for retaining excellent compaction. Use water when compacting. If you really want to invest in the life of your concrete drive install a wire mesh in the concrete 2" from the surface and place a vapor barrier over the compacted sub base. Create a thickened edge on both sides of the driveway. If your concrete drive is 6" thick the concrete thickened edge should be 12" thick by at least one foot of each edge of the driveway.