I know a lot of people use the party cup into a Air-pot, but isn't that the opposite of what you want to do? You will have a ball of root mass that's in the cup when it could be air pruning in the pot?
J TaNG had the good idea of making an indent in the centre around the plant, so it makes water fall into the centre, I've been doing this and it works great.
You just have to water slower and take you time.