The tongue position is an often overlooked but critical aspect of good horn playing, and goes well beyond just tonguing the notes.
I encourage all horn players to incorporate lip slurs into their daily routine, as smooth lip slurs require good tongue arching. For a smooth upward slur, from a 3rd space C to a 5th line F, the tongue will arch from a "taah" position to an "ee" position in the mouth.
Check out this great MRI footage of how the tongue functions in a variety of horn playing settings (rapid tonguing, double tonguing, scales, slurring, etc) that Sarah Willis of the Berlin Philharmonic has provided. Note how the tip of the tongue is employed for tonguing, but the entire tongue's position changes depending on the range played.
Especially, check out how much her tongue arches for higher pitches!
Horn teacher and performer