If you actually are *using* the excerpt field in posts, just place the_excerpt()
template tag on a line in your theme templates (inxed.php, single.php, etc.) just above where the_content()
resides. For example, here’s a mod of the default theme’s index.php:
<div class="entry">
<?php the_excerpt(); ?>
<?php the_content('Read the rest of this entry »'); ?>
</div>
If you don’t want it to display when no excerpt exists (if not, it repeats the first 120 words or so of a post’s content):
<div class="entry">
<?php if($post->post_excerpt) the_excerpt(); ?>
<?php the_content('Read the rest of this entry »'); ?>
</div>