Here's a simple exercise for building tone and legato technique. I've used it in a number of lessons. It's also nice for warming up.
It seems so simple but it can really help. Here's the point: I find many students can blow with a nice resonant tone until they begin to tongue. The air flows happily from lungs to lips until that tongue gets in the way and fouls things up. The tongue is like some bureaucrat setting up a checkpoint saying, "Hold on just a second here. Before you [air] can get to the lips, you'll need to sign these forms in triplicate."
Meanwhile the poor lips are starved for air and just can't vibrate.
Here's the trick to this exercise: start that half note with beautiful flowing air. Now (here's the key) when you start legato tonguing, use the lightest stroke of the tongue possible. Use a light, minimal "d" and make sure the "ah" is unbroken (well, minimally broken).
Listen carefully to your sound. Does it lose fullness when the tonguing starts?
Keep it open. Keep it resonant. Remind that tongue "bureaucrat" that he just isn't that important in the scheme of things.
We really shouldn't use the term "tonguing" at all. Just gives the tongue an inflated ego.
"Look at me. Look at me. I'm the tongue. I'm so important. I deserve the spotlight."