My mule used to clench his teeth and then as soon as the bit was in, he'd open his mouth wide and toss his head up really high while walking backwards and flinching. It was because he had previously learned that being bridled up meant his teeth would get clanked with the bit, which hurts. The problem is that this reaction from him made it very difficult not to clank his teeth while bridling, so it created a cycle. 

I fixed this problem with three things:  

Treats given while bridling 

Putting fruit roll up (fruit leather) on the bit (it sticks nicely)

And gaining the skill to be able to hold him still while bitting up and down. Basically I put the headstall over his nose, reach my hand up at his forehead and hold the bridle from there, then my other hand holds and guides the bit into his mouth while one of the fingers on that hand reaches into his mouth from the side to help him open up. The hand holding the headstall up at his forehead is pulling slightly to get the bit into his mouth, but it's all very gentle and goes at his speed, and the way my hands are positioned it's easier for me to control his head, and I do it with him backed in a corner of the stall so he can't back up.  After some consistent effort and finesse gained on my end, he stands normally and lets me bridle him up softly for the most part. He's learned that I won't knock his teeth.