WordPress documentation is all over the press, so it took me awhile to actually locate this. In general, I display the full text of every post on the front page of my blog. The liveblogging I did of a conference this past weekend, however, led to huge posts (4-5K words) being displayed in full on the front page.
After a bit of searching, it turns out you can insert the a <!–more–> tag in a post and then on the front page view, WordPress will cut off the post at that point and put in a “Read More” link that directs the reader to the archive of the full post.
One of the drawbacks of doing this is that the RSS feed will also only show the truncated version, but there are plugins that will override this behavior and force a full text feed regardless.