The appropriate way to make policy systematic, or rule-like, is to base policy decisions on the state of the economy. That is, policymakers should describe the reaction function that determines how the current and future policy rate will be set depending on the state of the economy.
Policymakers are, of course, no more certain about future economic conditions than anyone else is, and therefore cannot realistically commit to particular future values of the policy rate. Nonetheless, describing a reaction function or rule that explains how the policy rate will be determined in the future as a function of economic conditions can be highly informative.