If you're using WordPress 2.0 you may notice one of the cool new features is the post preview. When you save a post or edit a post down at the bottom of the post page you are presented with a preview of your post. This preview shows the whole page (not just the post text), logo, navigation, everything that is on the main page. I really like this feature but I don't always want to use it. Most of the time I don't need to look at it. For example when making a simple change. So it is just a waist of bandwidth and server power. If you read my blog regularly I'm sorry you know that I never just accept these types of things. So I modified the /wp-admin/post.php file to allow me to view the preview only when I need it. I considered making this a plugin but as far as I can tell there is no way of preventing the preview page from loading via a plugin. I could easily hide it but the data will still be pulled from the server. So if you would like to make the changes yourself open /wp-admin/post.php and find line 85 

<iframe src="<?php the_permalink(); ?>" width="100%" height="600" ></iframe>

and replace it with 


<script type="text/javascript">
<!--
  function show_preview() {
    var elm = document.getElementById("frm_preview");
    elm.src = "<?php the_permalink(); ?>";
    elm.height = "600";
    return false;
  }
-->
</script>
<a href="#preview-post" onclick="show_preview();">Show Preview</a>
<iframe id="frm_preview" src="" width="100%" height="10" ></iframe>

I've posted this to the official WordPress tracker for possible inclusion in the next release.

* Edit for 2.0.1*
If you are using WordPress version 2.0.1 then you should replace line 83 in /wp-admin/post.php with: 


<script type="text/javascript">
<!--
  function show_preview() {
    var elm = document.getElementById("frm_preview");
    elm.src = "<?php echo add_query_arg('preview', 'true', get_permalink($post->ID)); ?>";
    elm.height = "600";
    return false;
  }
-->
</script>
<a href="#preview-post" onclick="show_preview();">Show Preview</a>
<iframe id="frm_preview" src="" width="100%" height="10" ></iframe>
* Edit for 2.1 *
If you are using WordPress version 2.1 then you should replace line 72 in /wp-admin/post.php with: 

<script type="text/javascript">
<!--
  function show_preview() {
    var elm = document.getElementById("frm_preview");
    elm.src = "<?php echo attribute_escape(apply_filters('preview_post_link', add_query_arg('preview', 'true', get_permalink($post->ID)))); ?>";
    elm.height = "600";
    return false;
  }
-->
</script>
<a href="#preview-post" onclick="show_preview();">Show Preview</a>
<iframe id="frm_preview" src="" width="100%" height="10" ></iframe>
* Edit for 2.2 *
Preview is no longer inline so this change is not needed.