UPDATE: This functionality is now native to the revised BLOGGER editor. Check here!
I know how painful it is to have to scroll down five kilometres to read a post, so what I've done is follow some useful tips online to determine how to expand a post. This guy's site is amazing for this sort of information. I doubt he's even - you know - foreign.
Check out http://www.blogbulk.com/2008/11/read-more-or-expandable-post-summaries.html