Cron Magento

Need somebody that really knows how to make cron job work on magento 1.4. My pricerules dont stick, display of product are perfect, whit pricerule allways, but when the items are in the cart, the pricerule stop to work minimum once a day, and i have to run the pricerules manually. You know 100% what to do, dont bid if your unsure about it. Identical cron job has to be placed on 2 magetno shops a .dk and .com