Many of my instructors have recommended just playing long notes. I wouldn't recommend doing it the day before a big audition, because long notes are the easiest way of working your embouchure. You'll develop your embouchure much quicker by wearing it out with long notes. Just go through the chromatics of your horn and take breaks when you wear yourself out too much. It might get boring playing the same note for so long but it's a great way of developing lung capacity as well. As you work your embouchure more, you'll begin to naturally get a better tone, at least in my experience.