const CACHE_TAGS_DATA_KEY = 'cache_tags';

    /**
     * Add tag to block
     *
     * @param string|array $tag
     * @return Mage_Core_Block_Abstract
     */
    public function addCacheTag($tag)
    {
        $tag = is_array($tag) ? $tag : array($tag);
        $tags = !$this->hasData(self::CACHE_TAGS_DATA_KEY) ?
            $tag : array_merge($this->getData(self::CACHE_TAGS_DATA_KEY), $tag);
        $this->setData(self::CACHE_TAGS_DATA_KEY, $tags);
        return $this;
    }