I just "got it" this year. I have been turkey hunting for four years. I bought diaphragm calls in the past, got discouraged and always put them away. Last year I felt I had gotten the yelp down pretty well, but that was about it.
I again this year decided to give it another try. I picked up the Primos CD that others have already mentioned. I am a police officer and work 6pm until 6am. Most nights are pretty dead after 3am. I pop in the CD in the squad and follow along. Yesterday I used only a mouth call and called in my first turkeys with it (3 hens and 2 jakes). I was pretty excited as this confirmed I have the sounds down well enough to fool the real thing.
My suggestion is number one pick up this CD. As others have said play it away from others, my wife got irritated with me practicing in the house pretty quick. The set comes with 3 different style/sized calls. Keep the call in your mouth and play with it and you will find the right spot where it should go. Find the call that works/fits you best (I found I can use the regular ones fine, my brother has his best sound with the a-frame).
It's easy to get discouraged and put the call away thinking you just cant do it. I would suggest when you get discouraged put it away for a day or 2 and then get back on it. I guess I am lucky enough to have a squad to myself and 3 boring hours a night to practice. Just keep trying.
I was able to pick up the yelp pretty easy. Once I learned to cluck experimenting with different things with my lips and force behind the sound I found I picked up the rest really quickly (though I still cant purr with it). The cluck and cutt are all in the lips and force coming from the lungs.
Once you get it it's like riding a bike, you know right where to put the call in your mouth, the force needed to make the sound you want, what to do with your lips and how to control rythm and volume. I am now trying out different calls with all the various reeds and cuts to find the ones I like best. So far I have found different ones produce different sounds I like but havent found one I can use for everything. One makes a great kee kee, one makes a great cutt etc.
One other tip. GET THE CD and follow along, but I did find I use different reference words from what the CD offers. For example they reference the word "pluck" to cluck. I experimented and found I get the best result for me using "pup" as a reference. Experiment and then practice and you'll get it.