The student and music director should work together to make the music exciting, meaningful and challenging. Practice more than just your ensemble music. Make solos, etudes, and other technical material such as scales a daily part of your practice time. A beautiful sound is the most important trait of a good tuba or euphonium player. Listen to recordings of professional performers for inspiration.
Air is fuel for a brass instrument.
Our instruments need plenty. Buzzing your mouthpiece away from the instrument will strengthen the lips, relax the breathing, and improve your flexbilty. Buzz scales, tunes, and even your music. Take private lessons for more individual help. Discipline yourself to a set a specific amount of practice time each day.