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Let us assume we are working on a "trill" between c in the staff and e (top space). First, we are going to mess with the rhythm and have a series of grace notes and longer ones--"c-eee-c-eee-c-eee (all slurred, of course)" and the opposite--"e-ccc-e-ccc-e-ccc (here 'ccc' means a longer c)." This trains the instant movement from one note to the other. This is millesecond stuff, and it is o.k. to break it down.
Wait, there is more!
Which was harder" "e-ccc," or "c-eee?" For most folk, slurring upwards is a bit more difficult (gravity and all [unless the composer makes you skip harmonics going down,]) and we come across the issue of where "home" is, which is usually the lower note.
Consider a person with two intimate friends. Ideally they will live between the two friends, so that each is an equal distance apart. (Being Politically Correct for The Brethren is such a pain!) Going chromatically between our c and e, somewhere around d is middle ground, Here we can invent two sub-sets of exercises:
Play "d,e,d,c,d,e..." until you can do it real fast, then leave the "d" out.
Exercise two has us "buzzing" a "d" on the mouthpiece, memorizing the "feel", maintaining that "feel," and using that as a "lock in" point--not the "c," not the "e." Home lies between the two.
Third and last trick, stolen from Denis Wick: Play the "trill" in slow motion, with a lip glissando between the notes, and note where the change between notes takes place. Nothing magical other than awareness.
Do these things along with your other exercises and wait for the miracle.