I'm an expert on this, it has been drilled into my head over the past few weeks.
Start off with playing scales, which ever one you like, chromatic scales are good for this, (this is also a good way to learn scales) every note lasting for eight crotchets, at tempo 50, make sure you really listen to the tone you're producing and play about with you embouchure etc.
Then, do the same only start off every note very quiet (
pp) and become very loud (
ff), and then the next note very quiet becoming very loud etc.
Then, do the opposite and start off every note very loud and and gradually become very quiet.
After that, change it so that each note last for 12 crotchets (still at tempo 50) and start off very quiet, become very loud in the middle and then very quiet again for the end.
Then do broken chords (e.g. for C Major, C E G, E G C, G C E etc.) (tempo 50), every note lasting for 1 crotchet, (slurred tongued etc. have a variety), and make every note equal so that no note is more noticable than the others.
Do this as often as possible; every time you practice preferably, and your tone should hopefully start to improve!
Now you all see why I lost my enthusiasm for practice when my teacher told me to work on tone!