Do not cache custom block I have a custom block created in code. I do not want to cache the block. I know I need to use BLOCK_NO_CACHE but I am not sure where to use it.

function mymod_block($op = 'list', $delta = 0)
{
    // set up an empty array which will contain the block contents
    $block = array();
    if ($op == "list") {
        // Generate listing of blocks from this module, for the admin/block page
        $block[0]["info"] = t('mymod');
    }
    else if ($op == 'view') {        
        $block['content'] = "hello world";
    }
    // return the built content
    return $block;
}
In your list $op, add:

$block[0]['cache'] = BLOCK_NO_CACHE;


However, note that Drupal 6 will not automatically update your block information, you will have to do that yourself, see privatemsg_update_6002() for an example.