<div id="gridContainer">
<?php
$c = 1; //init counter
$bpr = 3; //boxes per row
query_posts(
    array_merge(
        $wp_query->query,
        array(
            'posts_per_page' => 6 //put number of posts per page here
        )
    )
);
if(have_posts()) :
    while(have_posts()) :
        the_post();
?>
            <div class="post" id="post-<?php the_ID(); ?>">
                <h1 class="title"><a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php the_title(); ?></a></h1>
                <div class="postImage">
                    <a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php the_post_thumbnail('grid-post-image'); ?></a>
                </div>
                <div class="postExcerpt">
                    <?php the_excerpt(); ?>
                </div>
            </div>
<?php
if($c == $bpr) :
?>
<div class="clr"></div>
<?php
$c = 0;
endif;
?>
<?php
        $c++;
    endwhile;
endif;
?>
<div class="clr"></div>
</div>