1) The fastest way to improve is:
PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE...
2) When warming up work on long tones, which means holding any note for as long as you possibly can while keeping the same pitch and tone.
3) Work on lip slurs, meaning playing different notes with out tongueing each one individually.
4) Using only the mouthpiece play familiar songs. This is called buzzing. It will help in getting a better sound and tone.
5) Learn and memorizing major and minor scales will enable you to get used to the pitch of the instrument.
6) When playing in a larger group concentrate on listening to others along with yourself to play in tune and balance.
These along with some other things I didn't mention will help. Good luck to everyone in improving!!