Sit up straight. In fact, it's easiest to play with a good tone while standing. Make sure your back is straight up and down when you are sitting and don't slouch! Turn your body at a slight angle so that you don't have to reposition your neck to see the music stand clearly.
Hold your instrument in the proper position. You've probably been told this at least a thousand times, but it can affect your tone if you hold it too low. Don’t collapse your stomach area and cannot take a proper breath or support the tone.
Breathe correctly. Take a deep breath with your belly, not your upper torso, before playing. You should see your belly physically expand.
Listen to your tone. You can probably fix things yourself. Just think about sounding resonant and full. Imagine that you are vibrating the whole length of your instrument.
Remember to support the air stream. In-tune notes always sound better, mainly in ensemble settings.
Know that better tones on certain notes will come with time and familiarity. As you become more familiar with lower and higher notes on the instrument, your tone will improve on them as well. Don't just assume that time will solve everything however. It takes practice too!
Use vibrato on longer notes. Vibrato is a technique in which the player pitch bends very fast. Act like you are whispering "ha, ha, ha" and try to play a note. It might sound unnatural at first, so practice until it is natural and even. Vibrato is a good technique that will create interest on longer notes and mask tuning inconsistencies as well. The speed of the vibrato depends upon the effect you are trying to achieve; a faster vibrato often portrays a more intense feeling, while a slower vibrato is more soothing.
Have your instrument serviced regularly. An instrument that works better has a better tone. Period. If a key is leaking, the rods are misaligned, or your instrument is ailing in any other way, that will negatively affect your tone. Have your instrument checked at a music store.
Get a better instrument. The quality of the metal (whether nickel, silver, or even gold) and the craftsmanship that went into making a wind drastically affect its tone. If you have a beginner's instrument consider buying a new one. Professional and intermediate instruments are crafted more precisely, and it is easier to get a good tone on many notes. Paraphrased from Wikihow