Ok, sooooo, I am a high schooler, and I used to be fairly good at playing my flute/piccolo. Now I can be fairly fast at tonguing on my flute, and a moderate on my piccolo... however, I have not gotten into the subjects of double and triple-tonguing.... I would love to learn how, but whenever my mouth goes to learn the vowels, or consonants, they get stuck in my mouth and feel awkward... I do have a fairly large solo in our upcoming concert. We got the piece just today (Holst's Second Suite in F) and I would like to try to learn double-tonguing fast so that I can practice this technique for longer and hopefully perfect it before our concert.... any tips?!?

Double and triple tounging are both difficult techniques, and in my own experience can take many months to perfect. I would experiment with different sounds . " tu ku tu ku", "ta ka ta ka", ti ki ti ki, du gu du gu, da ga da ga, di gi di gi" and see which works the best personally I like taka, tuku, and daga. Triple tounging you just repeat the first vowel so, da ga da. My advice for practicing would just be to start really slow at quarter note =60 or less then gradually work your way up. Also whenever you're just walking around or whatever practice without your flute by just saying the vowel sounds. Another useful technique is reversing the sounds so you get, more ka ta ka ta, this makes it more even, also just do scales with the throat or "ka" sound. The throat is the more unused of the two so unless you practice it by itself as well you'll get an uneven tone. I've played the holtz piece you're playing and its alot of fun but yes double tounging does make it alot easier. Your best friend is your metronome remember that.
Best of luck.

Don't try to practice long phrases of double or triple tonguings at once, or you'll get tired out very quickly. Practice a few beats at a time, rest for a beat, and practice a few more beats. If your tongue starts getting tired, increase the rest by another beat or two. Don't do this for long periods of time either. You don't have to spend an hour a day on tonguings. Just spend 5-10 minutes at a time on it, but you can revisit it several times a day if you want.