Help with cache-proxies

Hi to all, i have a big problem, i use a phpBB forum in my page, here in Spain the ISP's use a cache proxie system. And when change anything in my forum such as image or new functionality, my users can not view the forum. Yesterday i include a visual system confirmation to register, and today my users can not view the forum. This is the header directives of the header file of the forum if (!empty($_SERVER['SERVER_SOFTWARE']) && strstr($_SERVER ['SERVER_SOFTWARE'], 'Apache/2')) { header ('Cache-Control: no-cache, pre-check=0, post-check=0'); echo "Apache 2"; } else { header ('Cache-Control: private, pre-check=0, post-check=0, max-age=0'); echo "Apache"; } header ('Expires: 1'); header ('Pragma: no-cache'); Any one can help me Excuse my broken english Un saludo, Danny