A number of people have expressed an interest in the types of isometric exercises that I recommend for use when you must be away from the horn.

Here are a few ideas which I hope you'll find to be useful in maintaining and developing your chops when you can't take the time to practice:
 

1. The first exercise consists of merely holding your lips closed, as in saying the *mmm* sound.  When done correctly, this requires you to slightly roll your lips inward... Hold this position for as long as possible(you'll eventually be able to do it for hours at a time), until your muscles begin to burn, then rest an equivalent amount of time before repeating.
2. The second exercise is like the first, except that this time, besides the *mmm* position, you should also draw the lip muscles in, toward the center of your lips(avoid an obvious pursing, however). You should feel as though your lip muscles are *hugging* against your teeth.  It is also important to keep the corners where they are when your mouth is relaxed while you are doing this(neither stretched outward into a *smile* nor drawn down as in a frown). Once again, hold until the muscles develop the lactic acid *burn*, then release, rest, and repeat...
3. This one is best done before a mirror, at least the first few times that you perform it, until you are sure that you are correctly performing the exercise.  While observing yourself in the mirror, complete the following movements:

a).  Roll your bottom lip out as far as possible.(Try to touch your chin with it).  Be sure to keep the upper lip in contact with the inside of the bottom lip as you are doing this.  Hold in the extended postion for a count of ten, then gradually roll the bottom lip back up, so that it is hugging the outside of the upper lip. Rest. Repeat.

b).  Purse your lips as far forward as possible.  Hold for a count of ten, then gradually relax them.  Rest. Repeat.

c).  Roll your lips inward, so that the red(membrane) of your lips disappears.  Be sure that both lips are in front of the teeth as you do this.  Hold for a count of ten. Rest. Repeat.

There are other exercises which are also beneficial, including that of bouncing a small paper tube between your lips(but, again, in front of your teeth--gripping the paper only between your lips)...With practice, you'll eventually be able to do this with a pencil or pen...

Finally, there is an exercise which I do all day long (when I'm not playing) which is difficult to describe with words, but creates a sort of a squawk when done correctly....Roll your lips slightly inward, then use your tongue to force air through them, stopping the air by having your tongue close against the roof of your mouth....With practice, it's possible to play chromatic scales this way, and it's a good way to prevent the dreaded *braccck* attacks, since your lips become more accustomed to placing pitches very accurately....

(this is based on a post Jeanne sent to TPIN)