No one ever said overblow playing was easy.
One thing most people will agree on is that overblow playing requires a lot of practice.
So what do you practice to get to a point where you can incorporate overblows into your playing and more importantly, how should you practice?
One thing that really helps when you are learning to play overblows is to have 2 old harmonicas where the reeds have been taped shut. One with the drawreeds taped shut and one with the blowreeds taped shut.
On these you can easily practice the different parts of the overblow technique. Use the harmonica with the tape on the drawreeds to practice bending the blowreeds down untill they choke and use the one with the blowreeds taped to get the drawreed to respond to you when you are blowing into the hole.
You will find that the seperate parts of the overblow are relatively easy. Putting them together is hard, but becomes a lot easier when you have a clear feeling for the different parts of the mouth that are involved when doing these seperate elements of the overblow.