Practice reading out loud (but with someone helping to make sure that the student is saying the words correctly) is a GREAT way to improve vocal and pronunciation skills. Also, if the student can make a tape of himself or herself speaking the words correctly then practice with the tape, this will also help-. All in all, there is no better way to learn to speak better than to speak as often as possible. If the issue is in enunciation, also add in some vocal warm ups like "red leather yellow leather" "unique new york" and other tongue exercises to work on sounds.
The biggest thing is making students say words correctly, don't let them get away with mispronouncing words. Whether it is in the classroom, lunchroom or hallway if they mispronounce the word make them correct it then. Practice makes perfect, but only if you practice it correctly!
Students pronounce words incorrectly all of the time. When I hear students pronounce words incorrectly I teach them the proper way to say the word. I explain to them how important it is to pronounce words correctly and they really respect that.
I also think that reading strengthens vocabulary and pronunciation. Seeing words repetitively helps to learn the words better.
When I was in college, I took several classes that addressed this issue. One was entitled "Voice and Diction," a theatre class; I've forgotten the other, but it was an English course. Both classes emphasized enunciation and pronuciation. Many of the exercises actually broke down each sound of a word, letter by letter. For example, the word/letter "I" actually comprises several sounds: "ah-ee." The letter "A" has an "ay-ee" sound, etc. You might try some limited exercises each day with explanations of how different letter combinations differ--"ough" in "through" and "rough," for example. It seems like this would be excellent for ESOL students as well as others who are poor readers or who have minor speech impediments.