David,

For those of you used to my spiel, this is going to be a repeat of most everything I have ever said, but I haven't said it in a while... :)

(Standard Disclaimer)
The following is offered in an as-is fashion and to be read and tried completely at the individuals fancy and risk.

Playing the trumpet is relatively easy when done right, but very hard when done wrong.  When played right, it sounds right. The converse is also true.  By definition, there is not right or wrong way to play, if the end product is right - ergo in this case, the ends always justifies the means.  But this means that you have a end in sight.

For some reason, most people tend to try running before they learn to crawl.  Playing the trumpet is a complex set of coordinations - like walking, that require many different parts of the body to work in concert.  If the desired trumpet task is a complex one - like playing a scale, before the task of creating a good sound on a single note has been tackled, then only stands to reason that no notes on the scale will sound good in a consistent fashion.

The second problem is that people are under the false assumption that you can play the trumpet without breathing.  The trumpet is a wind instrument and requires wind to work.  The way your body is set up, if
you inhale deeply, and let go, you will exhale with gusto.  This is plenty of air to play the trumpet in to the middle register (top of the staff) and somewhat beyond at most dynamics.  The problem then becomes two-fold.

One: how do you get a big breath and Two: how do you stay relaxed while breathing that deep.

These points are interrelated.  Most people with problems playing tend to share many common "programming" errors in their trumpet program.  One is to breath in and hold the air before making the attack.  This is usually to set the mpc on the face or some other routine - but what results is trapping the air in the lungs and creating an explosive pressure situation - but also putting a crimp in the breathing apparatus - - usually at the throat of upper chest.  Air must flow in order to get the trumpet to work.  There are few pedagogues who would disagree.

Most people breathe deep while lying on their backs.  This is a natural process hard-wired into the body from birth.  The stomach area should rise and fall with each breath.  You can practice with an Arban book (or some other large (not too heavy) object on your stomach.

Posture is important.  Poor posture forces muscles to work to lift parts of the body out of the way so that then lungs can fill.  You want to have the chest raised (almost an "Attention" type pose) comfortably and it should REMAIN UP.  Check out the Claude Gordon book for more explanation (don't remember the title - something like Trumpet Playing is no harder than Deep Breathing).  This keeps the torso such in a position that the lower abs can be used to blow with and discourages using the upper abs, upper rib cage, and pectorals.  Since we usually breathe only enough to live, you will probably feel a stretching of the ribcage muscles when taking full breaths.  You may want to invest in a "breathing bag" apparatus to help you practice taking full breaths, since doing so with fresh oxygen repeatedly will lead to hyperventilation :) Then, once you get the air in, let it out.  Do not pause between inhale and exhale "for now".  If you hear yourself slurping air, or a hissing effect when breathing in, you are probably causing resistance to the inhale by having your mouth cavity too small.
Open the inside of your mouth more - use a long "O" vowel mouth size.

Be sure that you are listening to a mentally generated (also referred to as visualization) idea of the perfect trumpet sound in your head before playing the note.  Hear it in all its glory and detail.  Attack, duration, quality, dynamic, vibrato (or not), and release.  Let your body respond to the mental demand of "sound".  Just as a child learns to walk, your body will take the ball and run with it.  It will move muscles and adjust parts of your playing to cause this mental ideal to be produced.  But use the KISS approach (Keep It Simple Stupid :) - start with single notes.  Then graduate to moving notes, slowly.  Demand the highest quality on your practice sound.  Make music, not just a noise, but a joyful noise!

Now to your question, you blow air.  The amount of pressure needed is the amount to keep air from escaping around the lips and mpc.  More tends to cut the supply of blood off from the lips and end your ability to control the sound or pitch.  As I said earlier, this is an extremely complex set of activities and it takes time to allow the body to learn to coordinate these activities.  Why most people get into high mpc pressure situations is to compensate for either not enough lip strength, incorrect aperture, or inadequate air - and they do this to play higher than they can.  This does not mean that you shouldn't chip away at the top of your range each day, but don't make it more than about 5% of your total practice.  Quality sound and ease of production is the goal - do this right, and the uppers should start appearing.

Over-analyzing muscle movements can also lead to problems due to the fact that this process is too complex for you to consciously focus on all its aspects at once.  The brain can, if allowed.  But it will coordinate all aspects of the apparatus - to create the desired result.
If there is a problem in one of more parts of the chain, a little practice on those parts can really help, but it needs to be kept in perspective.

One other things that I have learned recently.  The lips need to be placed in loose relaxed contact with each other prior to starting a pitch.  The aperture is created by blowing this apart.  They lips should be relaxed and not tight - this will lead to a pinched sound.  Remember this is a blowing motion - like blowing on a candle flame or blowing like trying to keep a feather or balloon in the air.

You need a qualified private instructor to listen and offer suggestions as to what in your sound needs changing.  A good instructor is trained to hear things in trumpet sounds that most people are not.  Either find one (best), or use a tape recorder to tape yourself and sent it to one of us {or by phone} (second best), or tape yourself and listen very critically.  You have to ask yourself, "How would 'insert-your-favorite-trumpet-player-here' sound doing this?"  "Would 'he/she/it' sound like this?" "What is different in my sound from theirs?"

These are beginning points of a grand adventure.  To paraphrase the great MF, you never learn how to play the trumpet.  It is a life-long venture full of surprises and revelations.  Enjoy.

I hope this helps.  I'm sure most all of this can be said more succinctly and probably more clearly - but that is not one of my gifts :)

Tim Phillips
iplatrpt@twave.net
Hudson, NC (the middle of rural Nowhere, USA)