[Note: I originally posted the two suggestions below into a a different thread where they did not really fit. I'm moving them here.]
Tonguing is a topic that comes up often on the forum. It can be a very complex and frustrating topic, but let me propose a couple of ways to improve tonguing technique that both use my favorite concept of "follow the sound."
First tip: improving your multiple-tonguing technique:
If your tongue is in the right place, the sound will be better. And of course, if you can make your tonguing sound better, then your tongue position is moving in the right direction.
Check out this blog post on the topic:
http://www.dwerden.com/forum/entry.p...riple-Tonguing
Second tip: maintaining quality sound in different registers while tonguing:
Your intended question is actually a little harder to answer! The idea is to get air moving through smoothly with sufficient quantity to produce good tone while tonguing.
There are at least two things to work on:
First, on a foundational basis:
You need to work on your strength and fluidity in general in all ranges. None of us will tongue in an optimum way when we are working to just get the note out. If you want to sound proficient when tonguing on high Bb, for example, your range should extend a 3rd or 4th above that with strength. Then you won't be working all-out on the Bb and your tongue will have some flexibility to adjust itself.
Second, the actual process:
Think of your song like a little wind-up car moving across a table, and one which you need to stop briefly a few times along the way. You want to apply as little force as possible to achieve the result. That means you need to have good muscle control, which starts with strength (see point #1 above) - otherwise you might awkwardly bang on the car to stop it. Also, in anticipation of stopping it, you don't want to have your finger linger on the roof as the car rolls, because that will impede its progress. So your finger hear = your tongue when playing, and the car = your tone.
Start by playing a long tone, perhaps 2 whole-notes long, in the range you're working on. Keep the airway open and the sound pure and full. Do it again and this time separate the two whole notes with a very light attack. Try not to move your tongue until it is absolutely necessary, and then withdraw it quickly. You want the tongue to create the most subtle break possible at first. Then go for a clearer attack, but without compromising the airflow/tone. Once mastered, try to play 4 half notes that way, then 8 quarter notes. Your ear can help you with this, and with the more widely-spaced movements your "feel" can also help keep track of things. If you do this in the high range, and then try it in the low range, you'll find the physical efforts are quite different. So it can help you practice to get comfortable with the different techniques required in all different registers AND at various dynamics. Your embouchure needs to stay very stable (per point #1 and general practice) and your tongue needs to move with great accuracy and efficiency. If you do an exercise like I describe, it helps you focus first on tone, which will lead you to the right tongue position/shape.
Listen to a little of Doc Severinsen for an example of the kind of non-disruptive separation I'm talking about. You'll hear several examples in the first minute. One gets the impression he has a well-trained set of chops and a very accurate tongue (even if it is used a bit more heavily than a euphonium style might call for):
https://www.youtube.com/watch?v=HZPXKoR2Mek