What are some poetic devices that Shakespeare uses to enhance the effect of the witches' chants in "Macbeth"? (Act 4 scene 1)

Shakespeare relies on alliteration, rhyme, rhythm, and repetition in this scene.  Here is an explanation for each:

Alliteration - repetition of initial consonsonant sound.  Alliteration is the verbal equivalent of a drum beat, adding to the sense of rhythm that may or may not already exist in a passage.  Here is an example:  "toil and trouble";  also "boil and bubble".

Rhyme - the use of rhyme creates a song-like effect to a piece of writing.  This elevates the writing, making it some cases more romantic, in others more suspenseful.  In this case, it is suspenseful, accentuating and connecting each line to the other and - again - acting like a drum beat in the scene.  The rhyme scheme is aa/bb, meaning there are always two lines together that rhyme.

Rhythm - This is the versification of written language, so that the "beat" of the words is the same and repeats.  In this case, the rhythm is in iambic pentameter, meaning that the beat is "duh duh DA".  The beat builds the tension in the theme.

Repetition - Each of the listed techniques is a form of repetition.  However, Shakespeare also repeats the chorus of "double double toil and trouble/fire burn and cauldron bubble".  This enhances the idea of a "spell" and makes the scene more creepy.