To add emphasis to a normal verb, we use the emphatic "do":
- He does run fast.
- Do come in.
- Do brush your teeth.
Obviously, with modal verbs this would be a grave mistake:
- (*) He does can run.
- (*) You do may come in.
- (*) I do should brush my teeth.
Of course, you can just stress the modal verb itself or introduce an adverb. In the case of "should" you can also cheat by replacing it with "have to":
- He surely can run.
- You may come in.
- I do have to brush my teeth.
But are there other possibilities to add emphasis to a modal verb? Which solution is the most elegant? Do any manuals of style address this?