melody is what gives your words and sentences cadence. If you read your work aloud you will hear when you're getting it right - and when you're not. Here are a few devices and examples:
Assonance; the repetition of vowel sounds, like this : "On the fifteenth of May, In the Jungle of Nool, In the heat of the day, in the cool of the pool..." Dr. Seuss/Horton Hears a Who (my personal favorite).
Rhyme: an exact correspondence between the last syllables of words, like this: "There's too many kids in this tub. There's too many elbows to scrub. I just washed a behind, That I'm sure wasn't mine, There's too many kids in this tub. Shel Silverstein/A light in the Attic.
Alliteration; the repetition of the initial sound - one or more letters - of words in a phrase or sentence, like this: She sells sea shells down by the sea shore.
Onomatopoeia; literally meaning 'name-making' this is when the sound of the word defines it, like Jaberwocky and Bandersnatch (Louis Carol), the Shrike (Dan Simmons/Hyperion), or the Grinch (Dr. Seuss).
* Constance Hale/Sin and Sintax - another great book that will teach you 'how to craft wickedly effective prose.'