How do you guys prepare long posts for SE; particularly for those that support mathjax?
I write them in a plain text file (in my case with gedit or Geany) and translate with pandoc:
pandoc --mathjax -s -o post.html post.md
That gives you a pretty good idea of what to expect on SE, provided you don't use Markdown features one of pandoc and SE does not support.
I should maybe note that I keep a record of the images I create (in files) since they tend to be very time-consuming and sources don't end up on SE. I can only recommend using TikZ for this purpose, though, as the results are very nice. See here for details.
Then I started using stackedit (project), which works great, and integrates well with Google Docs/Drive and Dropbox. You can even install it as an App under google docs, which will create a stackedit document that will automatically save there.