Well, ideally, you wouldn't do that. This is the web. It's not like traditional publishing.
See, on the internet, we have this thing called separation of content from presentation. Your "content" is the text itself, with little to no formatting. It's generally supposed to be fairly barren. The "presentation" then describes how to display that content.
Why? Because when you separate the presentation from the content, then you can display the content in different ways. What if I don't want to see your content on the webpage? What if I use a feed reader? What if your content is syndicated to another website? The whole point is to make your content very plain, in order to enable it to be displayed anywhere and then presented according to where it is displayed.
Adding vertical blank space is more a function of presentation. It's also something you probably don't want to be displayed everywhere. What about in a feed? What about if the content is presented on another site (you never know when you will want to migrate your content elsewhere)?
So, the question really becomes not how to add this space, but why you want to add it. Is it to separate posts? Do you want the paragraphs to be further apart? Define what your goal here is, not how you wish to achieve that goal. Because what is the "right" way to do it highly depends on exactly what it is that you are trying to do.