linking - connecting sounds
schwa vowel sound
function words as opposed to content words
intonation patterns for individual words
intonation patterns for phrases and sentences
new information stress
And, of course, there are individual sounds of letters. Each student is different. However, there are some things in this area that students have in common, especially if we are talking about students whose first language is the same. Oftentimes, the focus is on minimal pairs when dealing with individual sounds, though it need not always be.
In addition to the points I've listed, learning to move one's lips and mouth to form the sounds of English better is another area in which to focus. But, as I said, each student is different, and not everyone will need to focus on this.
With some students, merely speaking a little louder contributes to improvement in their pronunciation.
Please, post any follow-up questions you might have.