The pitch is adjusted with lip position and the speed of air. They control the aperture size between lips, which sets the pitch. You can affect the air speed by raising and lowering the back/center part of your tongue. You can notice this by whistling high and low. Do not raise the tip of your tongue, this prevents the air flowing freely between your teeth.
Playing these exercises will train your lips to adapt to current pitch which helps you to hit the right notes. Slur (no tongue attack, except for the first note) the notes and use the same fingering throughout the whole phrase. Breathe in on the last beat of the last bar of each phrase, do not hold breath. Keep your posture straight yet relaxed. The first exercises are also good for warm-ups.
Remember to use the metronome.
Sound example, Flexibility 1, Tempo=60
Lip Flexibility Exercise 1
Sound example, Flexibility 2, Tempo=60
Lip Flexibility Exercise 2
Sound example, Flexibility 3, Tempo=60
Lip Flexibility Exercise 3
Sound example, Flexibility 4, Tempo=60
Lip Flexibility Exercise 4
Lip Trill Exercises take the same idea further, this is more like "fine-tuning". These are not as ideal as warm-ups as the previous exercises. You can change the pitch rapidly by raising and lowering your tongue as described at the top of the page.
Sound example, Lip Trill Exercise 1, Tempo=100
You can click the image to get a bigger view.
Sound example, Lip Trill Exercise 2, Tempo=100
Sound example, Lip Trill Exercise 3, Tempo=60
Next we'll go even further and try to leap over notes. Again, this will train your lips to hit the right notes. Remember to use the same fingering throughout the phrase and use no tonguing.
Sound example, Leaps 1, Tempo=60
This is a longer version of the previous exercise.
Sound example, Leaps 2, Tempo=60
Further studies: Colin, Charles: Advanced Lip Flexibilities