If you look in the formatting palette that runs across the top of the box--like the one I'm typing in now--you'll see (at the far right side) an icon of a sheet of paper with a rip across it. When you get to a point in your post that you want to break (you can do this after you've hit "Preview" and decided on a spot, then closed "Preview" to return to the text box), you put the cursor at the point you want the break. Then you click on that icon. You'll see a line appear across the text box. You can just hit "publish post" at that point, or of course you can always hit "preview" again. Of course you can also delete these.
I searched out how to do this and then went back through posts--since I'm a blog "administrator," I can do that--and inserted those breaks. Thanks for asking, Heather. And note that I've added a label here called "housekeeping." Let's use that when we have a "how to do this blogging thing" or a "blog etiquette" kind of question (as compared to a specific course-content question).