I have used Squarespace for web hosting and content management for a couple of years. There are many things I like about it, but writing large posts in the Squarespace editor is not one of them. I have used various programs to write posts offline and then paste them in to the editor. This leads to some annoying issues with formatting, for example repeatedly removing all of the blank lines in a post. It also adds all of the pasted text twice. If you look at the resulting HTML you see multiple div id="_mcePaste" tags.
I understand that other blogging platforms (e.g. Wordpress) also suffer from this problem. I have seen various clunky workarounds around the web, but nothing pleasant.
After a bit of searching and trial and error, I have a solution to this that really works for me. I use iA Writer to write the content in markdown format. In both the Squarespace iPad app and web interface, the editors have a markdown option. After selecting this, pasting the content from iA Writer works flawlessly.
iA Writer is a markdown text editor that features a minimalist interface. The intent is that you focus on the writing without being distracted by the interface. I use iA Writer on the iPad and the Mac and sync them via Dropbox. I really like the minimalist style, and it looks really crisp on the new iPad.
Increasingly I also like the idea of having content in an open, lightweight format like markdown. It has enough power to handle most of the formatting I care about in working documents, and is always going to be accessible across multiple platforms and tools.