cached-block.php

<?php

class My_Foo_Block_Thing extends Mage_Core_Block_Template
{
    protected $_cacheKey = "your-cache-key";

    protected function _construct()
    {
        $this->addData([
            'cache_lifetime'=> false,
            'cache_tags'    => [
                Mage_Core_Model_Store::CACHE_TAG,
                Mage_Cms_Model_Block::CACHE_TAG
            ]
        ]);

        parent::_construct();
    }

    /**
     * Get cache key informative items
     *
     * @return array
     */
    public function getCacheKeyInfo()
    {
        /** @var Mage_Customer_Model_Session $customerSession */
        $customerSession = Mage::getSingleton('customer/session');

        return [
            $this->_cacheKey,
            Mage::app()->getStore()->getId(),
            (int)Mage::app()->getStore()->isCurrentlySecure(),
            Mage::getDesign()->getPackageName(),
            Mage::getDesign()->getTheme('template'),
            $customerSession->isLoggedIn() ? $customerSession->getCustomerId() : 'guest'
        ];
    }

    /**
     * Retrieve child block HTML, sorted by default
     *
     * @param   string $name
     * @param   boolean $useCache
     * @return  string
     */

    public function getChildHtml($name='', $useCache=true, $sorted=true)
    {
        return parent::getChildHtml($name, $useCache, $sorted);
    }

}