Styling odd and even numbered posts differently

Every theme will be different, so I’ll make the assumption that your theme has something like this:

<div class="post">

First, you’d replace that with this:

<div class="post <?php my_post_class(); ?>">

Then, add this PHP to your functions.php file:

function my_post_class() {
	global $post_num;

	if ( ++$post_num % 2 )
		$class = 'even';
	else
		$class = 'odd';

	echo $class;
}

There are several methods to use now though because WP has its new post_class().