I've been trying to learn Arabic. This is a direct quote from my manual, in the section on pronunciation.
ayn. We have a muscle in our throat which is never used except in vomiting. Think about that and pretend you are about to be sick. You will find that what is normally called in English gagging is actually a restriction in the deep part of the throat. If you begin to gag, and then immediately relax the muscles in order to release the airstream from the lungs, you will have produced a perfect ayn. This sound must be distinguished from the glottal stop hamzah as the difference affects the meaning.
The next time you're being sick, be careful what you say.