How to prevent pages from automatically adding line breaks?

Try using get_the_content(); in your page template loop instead of the_content();

EDIT 05/24/2012 – Try this:

<div id="main-content">
    <?php while( have_posts() ) : the_post() ?>
        <h1 id="pagetitle"><?php the_title(); ?></h1>
        <div class="pagecontent">
            <?php echo get_the_content(); ?>
        </div>
    <?php endwhile ?>   
</div>

P.S. You will also need to consider the rest of your more “normal” wordpress pages through out your site.

You don’t want ALL of your pages functioning like that. I would create a page template for the pagess that need to function this way, the rest of your pages should use the normal page.php template.

Learn more on page templates.