My general checkbox meta box for the WordPress admin area. All custom fields prefix with custom_ are saved automatically with my update function.

functions-general.php

<?php
function post_type_custom_fields_general_featured() {
	global $post;

	$custom		= get_post_custom($post->ID);
	$is_featured	= is_custom_boolean($custom, 'custom_is_featured');

	echo '<p class="checkbox" style="padding-top: 5px;">';
	echo '<input type="hidden" name="custom_is_featured" value="false" />';
	echo '<input style="margin-right: 5px;" id="custom_is_featured" value="true" type="checkbox" name="custom_is_featured"' . (($is_featured) ? ' checked="checked"' : '') . ' />';
	echo '<label for="custom_is_featured" style="font-weight: bold;">Set as Featured?</label>';
	echo '</p>';
}

function is_custom_boolean($custom, $key) {
	if(!empty($custom[$key][0]) && $custom[$key][0] === 'true') {
		return true;
	}
	return false;
}

functions-update.php

<?php
function post_type_custom_fields_update($post_id) {
	global $post;

	// check user permissions / capabilities
	if($_POST['post_type'] == 'page') {
		if(!current_user_can('edit_page', $post_id)) {
			return $post_id;
		}
	}
	else {
		if(!current_user_can('edit_post', $post_id)) {
			return $post_id;
		}
	}

	if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
		return $post_id;
	};

	// authentication passed, save data
	// cycle through each posted meta item and save
	// by default only saves custom fields which are prefixed with custom_
	foreach($_POST as $key => $value) {
		if(strpos($key, 'custom_') !== false) {
			$current_data	= get_post_meta($post_id, $key, true);
			$new_data	= !empty($_POST[$key]) ? $_POST[$key] : null;

			if(is_null($new_data)) {
				delete_post_meta($post_id, $key);
			}
			else {
				// add_post_meta is called if not already set
				// @see http://codex.wordpress.org/Function_Reference/update_post_meta
				
				if(strpos($key, 'custom_date_') !== false) {
					$new_data = strtotime($new_data); // convert to timestamp
				}
				
				update_post_meta($post_id, $key, $new_data, $current_data);
			}
		}
	}

	return $post_id;
}
add_action('save_post',	'post_type_custom_fields_update');