This paper proposes the concept of Key-resource/operation-place Pairs (KP) of S3PR (systems of simple sequential processes with resources). Based on KP, two policies are presented to prevent deadlocks in flexible manufacturing systems (FMS) that can be modeled by S3PR. The idea is to control some key resource places only to guarantee that all strict minimal siphons (SMS) never become empty, thereby making the controlled system live. It enables one to design two easy-to-implement control policies. The first one can guarantee that the controlled system is live, and the second one can also make the controlled system live if there is no SMS containing any control place in the controlled system. At last, a well-known FMS example is used to illustrate the proposed concept and policies.