Finally! Blogger has introduced After the Jump a feature which lets you create expandable post summaries in your blog posts, so longer posts appear as an intro with a link to Read More.
Creating jump breaks in your blog posts can be easily done right from the post editor, without the need for any HTML changes.
The only thing you have to do is to add this code into the post editor:
Take a look at this post that explains in detail how to implement expandable post summaries.