One of the best ways to improve a players handles is to do basketball drills that are going to get the player out of their comfort zone. That means that it is okay to turn the basketball, over or mess up when doing ball handling drills, it is actually even encouraged. You want to be going as hard and as fast as you can. So that way when you get into a game situation your handles are prepared for whatever you come across.
This two ball attack double crossover dribbling drill is going to work on making a move with two basketball while moving at a high speed forwards. The goal of the drill is to help the player to learn to be able to handle the basketball with both hands while on the move. Being able to make a move on the move like this simulates a transition scenario really well.