I have always had lots of problems with unclear attacks. There were several reasons:
I used to believe that I had no problem tonguing softly between the teeth. (This is the strongest reason why many teachers don't recommend it.) My teacher showed me that playing softly, the sound will start a little bit stronger at the attack. So playing softly with the tongue does not work too well. At louder levels, it works better.
It's good to learn both techniques and use them where appropriate.
Bill Carmichael demonstrates tonguing between the teeth in his video.
Hard tonguing in the higher register is a good strengthening exercise.
The tongue is a muscle that needs to be trained. Both endurance and speed must be trained. Usually, we remember to exercise for speed, but the endurance part is easily forgotten. To train tongue endurance, tonguing in the middle and lower register for long periods is helpful. A nice side effect of such practice is that it trains embouchure endurance as well.
Excellent tongue endurance exercises are found in the Gollehon book.
Copyright (c) Rune Aleksandersen 1997 - 2002