Working on articulation is part of my daily routine. There are really two parts to the 'act' of tonguing, the consonant and the vowel. Consonants can be more pointed with sounds in the 't' range, and softer with those in the 'd' range (both of these work well for single and double tonguing). Sometimes very soft piano attacks work well with 'p'. For the second syllable of double tonguing, 'g' for a more legato effect, or 'k' for a shorter style work well. The Vowel chosen can be an 'e' sound, as in the word free, or year, or 'a' sounds as in far...even an 'i' as in kick....but the key is to experiment and see which combinations work in the given musical circumstance....