One overall thought from recent auditions and horn performances I have heard is that tonguing is one of the most important technical areas to master. Which is why I appreciate in particular a very recent article by David Wilken with the title “Tonguing For Brass Playing.” What I especially like is he goes well beyond the typical description we see in horn sources and looks at the big picture of the huge variety of ways to approach tonguing that are seen among fine brass players.
While I am sure some would think that this type of broad look could lead only to an approach that bordered on over-analysis of minutia, I would instead propose that even if you think that some of the more extreme methods (i.e., very different than the typical) are totally wrong it is still good to have some understanding of them, it will help you sort out what you are actually doing (which is probably somewhat different than you may have perceived) and may also point to directions for change for yourself or your students.
There is certainly no one way to tongue notes on brass instruments and overall the approach to horn and tuba is a bit different than trumpet and trombone. One of the more extreme ways to tongue is an approach called the “tongue controlled embouchure.” I believe this is very rarely used among horn players (it is most often associated with the trumpet) and also it is an approach that is hard to even describe briefly. Wilken does a good job of keeping it concise (and in the context of a section on “The Tongue Tip While Slurring and Sustaining”) and provides an additional link to more information on the topic (included in the quote below).
There are a minority of players who keep their tongue tip touching their lower lip while slurring and sustaining pitches. I usually discourage this for the same reasons I discourage attacking the pitch with the tongue in contact with the lips, but this method often combined with the spit attack (this is how a “tongue controlled embouchure” is usually taught). There may be some players that find it necessary for the tongue to provide some support structure behind the lower lip, if they have a very large lower lip and short lower teeth, but I think this is probably quite rare. There are reasons why this method may help some players with the extreme upper register, but I think the drawbacks caused by the tongue in contact with the lips so much outweigh the benefits. Instead, I believe that most players who find a tongue controlled embouchure so helpful would do better by anchoring the tongue on the lower teeth or below and attacking the pitches with the so-called anchored tonguing I described above.
Returning to my initial thought at the beginning of this article, tonguing is really a big problem out there in the horn world. A lot of players don’t tongue well and it is actually the main technical problem they have not mastered. If you have problems in your tonguing (such as people keep telling you to play shorter or whatever) you may need to go outside the box of traditional horn study materials and things horn teachers typically say (often parroting the Farkas book without any further critical thought) and put some personal study into the topic. Visualizing short notes when you can’t play short notes won’t do the job. An article such as the one quoted here by David Wilken can be a starting point for such a study. I also have more thoughts that will get you started on the topic here.