In the index.php template files, look for the end of the postmetadata section and the final DIV of the WordPress Loop. When the page is actually generated, it looks like this:
</div>
<div class="post" id="post-2">
Now, the second line is the beginning of the Loop where the posts repeat themselves down your page. The DIV is the end of each post.
There, put a <hr /> tag (XHTML version) so when the page is generated, it would look like this:
</div>
<hr />
<div class="post" id="post-2">
There are other methods, but this is the easiest.