The Quality of Service (QoS) is one of most important issues in providing an acceptable level of service. Communication engineers need a guide for action to improve QoS parameters such as data rate, bit error rate, latency, and jitter. Improving QoS includes a set of steps that should be performed by an engineer; these steps are belonging to two categories, system and network steps. In this paper we developed a flow chart of a general guide to improve QoS in a digital communication system, based on our knowledge of the modern communication system and impairments that lead to system degradation. We outline the options and advices to approach the subject of QoS in digital transmission.