Things that've helped:
1) Using body pillows to force sleeping on side, which straightens the neck (versus face-down) — also lessens low-back pain
2) DENTAL GUARD (a 3D-printed plastic insert which form-fits my teeth, slightly opening jaw-angle/mouth)
3) Losing weight — YES, sleep apnea is worsened by excess weight
4) Pseudoephedrine (an hour before sleep) — but be careful cause too much and you won't ever sleep
YMMV; I am not a doctor (just fat guy with apnea).