In a text editor it is convenient to edit Markdown with hard line breaks. Some renderers (such as the one in Trello), however, turn these line breaks into <br> instead of flowing whole paragraphs together. To convert my file for these, I use ~/bin/mdflow:

#!/bin/bash

pandoc --wrap=none -f markdown -t markdown

You can compose it with pbtr to copy the text in the editor, run pbtr mdflow, paste it into Trello.