How can I create expandable post summaries?

The Quick Answer
Add tags to your posts to mark which parts you want hidden in the summary version.
Add CSS declarations to your template to hide those sections on the index and archive pages, but not on post pages.
With this trick, you can choose to display an arbitrary amount of text from the beginning of each post, as a teaser for the whole thing. Then users who want to read the rest of the post can click a link to see the full text. THis is handy if you have lots of long articles all on one page. Note that you'll need to have post enabled in order to make this feature work.

There are three ingredients that go into this feature: conditional CSS, a "read more" link for each post, and a modification for the posts that use this feature. You an find the links and tags and codes, and modifications for this at google help.