The exercise I use most for improving tone is long tones. I use
the same routine that is in Mark Van Cleaves' Maximizing Practice Vol 1.
This is a great routine, that is perfect for working on tone, delivery
of air, and ease of production. It basically starts with warming
on low C using expanding intervals. You play Low C with a full sound,
crescendo and decrescendo. Then, move to low B, and repeat. Then C#, Bb, D, A, Eb, etc.. down to low F# to F# on the first space.
After this, you do the same routine but start on G in the staff, expanding your way to low F#, and G above the staff...
This routine has worked very well for me and my students...