A wise man once told me, 'Tone is important. Its the first and last thing anyone will hear'. This man, the late Abram Wilson, had an effortlessly rich trumpet sound. The kind of sound that was impossible to ignore. His advice still resonates with me years later.
Think about it. You could have brilliant ideas but if they're played with a mediocre tone that's not engaging or pleasing to the ear then whatever you play will be lost and ignored. Since that concert I have been constantly conscious of developing on my tone as a sax player. I am nowhere near where I want to be, but I will share with you a few of my ideas on developing your sound.
Know where you want to be
Its important to know where you're trying to get to. Listen to saxophonists whose playing you enjoy and focus on their sound. What do you like about it? Is it the power and sharp articulation of players like Kenny Garrett that gets you going or the smoky tones of Lester Young? When you know what you're aiming for it is time to focus on emulating this sound in the practice room. This doesn't mean trying to sound identical to your favourite player! Having an example of what you want to sound like will give you a model you during practice. Doing wide and detailed listening to saxophone players will not only improve your awareness of different tones, it will also give you new ideas for improvisation and composition.
One of the most important practices for developing your saxophone sound are daily long tone exercises. You can find a full explanation of this exercise here. This exercise is a fundamental part of building tone for a number of wind instruments and is often overlooked because of its simplicity.
Another very useful set of exercises for improving your tone quality and control are overtone exercises. These are described in detail by Sigmund Rascher in his book Top Tones for Saxophone which can be purchased here. Having a mastery of the harmonics on the instrument will aid the development of a rich tone as well as opening the door to the altissimo register and the world of multiphonics.
Don't blame the gear
There's a story often shared by saxophonists of a young alto player in the 1940's who was determined to have the ideal tone. He went through a number of mouthpieces, countless reeds and a few different horns. He just couldn't find the perfect sound. One night at a jam, the great Charlie Parker asked to borrow his horn and made it sing. A sound like that was what the young altoist had wanted all along. It dawned on him that there was nothing wrong with his gear at all, the player was the problem...
So many players these days are constantly changing set ups. I'm not denying that good gear can definitely help create the sound you want and different mouthpieces and horns lend themselves to different sounds but it is important not to fixate on these things. Your tone is as much a product of your embouchure, air flow and aural imagination as anything else. Work on that and you will be able to make any horn sing.