Tones with Drones
The easiest and most prevalent method of developing a consistent sound is, of course, a regular practice of long tones. To add a practice of simple interval intonation to a daily long tone routine, it can be helpful to practice with a drone, so either grab a friend or fire up the Tonal Energy app and you can practice listening for tuning and sustain simultaneously. If using a drone generator, I recommend either using a bluetooth speaker or a single earbud; make sure you can still hear yourself clearly.
This is one example of adding drones to two common long tone studies (De La Sonorite by Marcel Moyse and Tone Exercises by James Galway). Download the PDF here.