You might first start off with just playing your reed only - leave the instrument out of the picture. Face a mirror, and first play long tones, watching your embouchure to make sure there isn't movement in the embouchure muscles, chin, etcetera.
Next, start tonguing very legato - in other words, keep a longtone of air going, then gently touch the reed with your tongue without ever stopping the airflow from your lungs. Don't cut the air off in your throat - just move your tongue. Make sure you're not moving the embochure as you move your tongue (what I call "chewing" the beginning of notes). A good idea is to play a continual line of halfnotes at a tempo of quarternote = 60.
When that's working well, start putting space between each halfnote (again at a tempo of quartnote = 60, play a half note, stop the air for a half note, repeat). Make sure that your long tone of air is moving, then stops in your chest/throat (without tensing up your throat!) Nothing in your embouchure should move at all when you stop the notes, nor should your tongue hit the reed. As Kent mentioned above, you are basically doing a very fast dimenuendo and stopping the air in your throat without tensing the throat. It's also important to make sure you're still using your abdominal muscles the entire time you're playing - don't relax your muscle support because you're ending a note. Often, people do this, then pinch with the embouchure to keep the pitch up.
If you find this difficult, try taking the reed completely out of the picture - just say the word "TAH" (rest) "TAH" (rest) and repeat, as if each syllable and each rest is worth two beats at a tempo of quarternote=60. When you put the reed in your mouth and blow, the sensation of stopping the air should feel just like it does when you end the word "TAH" and rest. It's basically an extremely rapid diminuendo, assisted with a feeling of stopping the air slightly in the throat.
Make sure to keep watching your embouchure. Doing this on reed alone will also help you hear if you're pinching at all (the pitch of the reed sound will alter if your lip pressure changes).
Once that's working well, increase the tempo gradually.
Then, add your instrument to the picture. Do the same process, picking first a note that is in the comfortable middle of your range. Then, try a low range note. Then, try a high range note. Then, play scales - very slowly, always watching in the mirror.
If you do this every day for a week or two, you'll find the chewing habit will disappear! It's a good idea to check in regularly with a mirror - sometimes we all pick up odd habits from time to time, and a visual check is a good way to identify a problem you might otherwise overlook.
Lynne Marie Flegg