2.The Dizzy Gillespie ‘puffy cheeks’ with your lips closed. Again dont overdue it. Just feel the stretch.Move the air around in your mouth to get all facial musculature engaged.
3. Buzz low notes/ pedal tones/ glissandos on either a trombone or tuba mouthpiece (real cheap on Amazon). Definitely gets the ‘ahh ooh’ going.I also sometimes do the Stamp/Thompson buzzing warm-ups this way before I use the trumpet mouthpiece to really engage the aperture corners
4. Of course you can always use finger massage. Stroke downward from the cheekbones toward your lower jaw.