Chris (and all TPIN'ers),
You've gotten a lot of responses (at least publicly) that say to work on "k" tonguing everything in order to speed up your double tongue. Well, I'm gonna go a different route...
I saw Chris Gekker in a master class in Louisiana this spring and the subject of multiple tonguing came up. His suggestion was to not worry about the multiple tongue technique but to instead work on slowly getting your SINGLE tongue to a relatively rapid level (he said he has his students work to ST 16th notes at quarter = 120). His theory (which he got from Ghitalla) is that once the ST is fast enough the double tongue (or triple) will be there.
My own take on this, especially after watching the Mr. Adam videos (see www.roth-music.com for more info), is that an extremely rapid single tongue forces the vowel pronunciation to be WAY forward in the mouth so that when you add the k syllable (or g) it doesn't get "caught" in the throat. Mr. Adam, in the video, points out on more than one occasion that we can all sing the opening to the 3rd mvmnt of the Hummel or the "Lone Ranger" (William Tell Ovt.--Allegro, for those on the other side of the pond) theme with ease. Why then do we have trouble when it comes to playing it? I think it's because we focus on the technical aspects rather than on maintaining our focus on the sound (MUSIC) that we are trying to achieve resulting in our air shutting down and without air the whole system falls apart.
Anyway, my $.02.