I wish I knew what to tell you; if I did I would bottle and sell it and make a fortune! I can really only tell you what I have done with my children (who are all different ages and maturity levels) with varying amounts of success.
The first thing is to acknowledge that you know that they want it now and that you know just how they feel. I have found that my children respond best when I empathize with them, even when what they are demanding is ridiculous. Often, once a child knows that the parent really does feel for them, the child will listen when he hears those dreaded words "Not now".
However, I find that there is another dimension that is equally important. The children have to see that their parents behave no differently with themselves. There is great educational value in children hearing from parents that the parents also want something but cannot allow themselves to have it immediately. That way the child will hopefully learn that the parent treats himself in the same way that the child is treated.
Having said all that I must admit that very often the child's personality is the most significant factor. Some children are able to accept the word "later" and some are just not.
