Step 1 - Set Proper Formatting
First of all, make sure you select all the text in your story (ctrl-A, or command-A) and then choose "Font Format" > "Paragraph" in the Story Editor window. This will ensure proper line-breaks and proper formatting.
Step 2 - Strip out hidden codes (copy-and-past plain text, only)
If you're story formatting looks odd after you've posted it, then it's probably because you've pasted Rich Text full of hidden font and paragraph codes. It's best to convert your story to Plain Text first and then copy and paste it in.
Text editors often have a menu item to allow you to do this. For example, if you open up TextEdit on a Mac, it will have a "convert to Plain Text" menu item. This will strip out all the hidden codes, before you copy-and-paste.
If you want to modify what you've already posted, then you can always go back to the story you've posted and choose "Edit". This will take you back into the story editor where you'll see your hidden codes.
Remove all the font information, paragraph tags, etc, and simply put in 1 or 2 line spaces in between paragraphs. You'll need to experiment to see what works best.
Hidden formatting will be in between two carrots. Often it will look something like this: <p> followed by an </p>.