You might want to teach your students outlining prior to writing. This will allow them to segregate their various ideas into individual paragraphs. I actually learned in a five paragraph approach (I'm taking this from a European history class which I credit for teaching me extemporaneous writing): Introduction, 3 supporting paragraphs, and a conclusion. This reduces rambling. Paragraphs can be long or short, depending on how much detail is necessary to support the specific subtopic. A 5 paragraph paper places a structure on the entire writing.