I write in plain text, but "publish" in HTML or print. I'm always paying attention to how my workflow treats certain punctuation marks when generating the final product:
Option 1: Type in ASCII, deferring the conversion of special characters.
The quickest thing to do while writing is just to type these characters the fast/conventional way: straight quotes instead of curly ones, two hyphens
-- for a dash, and three periods
. . . for an ellipses.
The problem then is to ensure these get properly converted during the publishing process.
Modern Markdown converters have an annoying habit of not offering a typography conversion layer (such as SmartyPants) or similar functionality.
I don't use WordPress, but I understand plugins are available which add a SmartyPants-type conversion layer. WordPress recently added native support for posts written in Markdown (plain text).
It should be possible to roll my own online dingus using the source published at the sites above, but I never got around to it.
Option 2: Use your text editor's auto-replace features to insert Unicode characters at the time of writing
This is the approach taken by Editorially -- when you type two hyphens and a space, for example the two hyphens are automatically replaced with an em-dash. Thus the typography is handled inline within the source text rather than being converted during the publishing process.
Other tools offering this option:
CTRL+SHIFT+'turns it on or off. Auto-replacements for dashes and ellipses can be supported by copying and pasting these characters into the Replacements tab in the preferences.
Doing it this way means you have to be marginally more careful about your tools -- specifically, both your text editor and your publishing platform have to able to handle your text in the UTF-8 character set.
For example, if you're publishing on the web and your text is stored in a MySQL database, you'd want to be sure it's using UTF-8.
You also need to be sure your text editor is saving your source text in UTF-8. Many editors have an option for forcing this. Others use UTF-8 natively (i.e., behind the scenes) but don't do a good job of documenting it anywhere.