There is no way to do this right now, at least not natively to the application.
Depending on how complex the formatting is, you might set up the post the way you want it, then go to HTML mode. Copy and paste the HTML into notepad and then save it. Next time you want to write a post, just open the text file and copy and paste it back into the HTML textbox of WLW.
Or you might try a plugin like this:
http://www.joecheng.com/code/DynamicTemplate/
-Brandon Turner [MSFT]