Play each overtone of the Bb overtone series including the fundamental (low Bb). Start on low Bb, play it for a minute taking as many breaths as needed, and then move up to the next and play it for a minute, and continue onward all the way up to the highest partial you can manage. I spend about 15 minutes doing this each day, which lets you know by the time I get to the top I'm pretty worn out and I don't have a minute of 4th octave Bb in me.
Most importantly, I try to make each note sound pleasant, but I've also incorporated other aspects into my long overtones like bends and vibrato.
For those of you who are wondering what an overtone is you should read some of my post on altissimo. Below is an approximate chart of the four octaves of the Bb overtone series: