Break the Matrix

I've found that biting down with my mouth full of a sturdy cloth with a focus on biting with the incisors has helped improve my gonial angle. My lower jaw is pretty retruded and doing this method naturally forces me to bring forward my lower jaw forward while putting the muscles under tension.

This should help if your mandible is retruded as well as if your maxilla is retruded.

I've also found that by jutting my lower jaw forward like kobe in this image the tongue will have more room to be placed on the roof of the mouth because my mandible will be in a more forward position, forcing my maxilla to play catch-up.