We study opportunistic multiuser communications, and propose two novel schemes to address scheduling in asymmetric channels and admission control in such systems, respectively. We first device a relay-Aided multiuser diversity (RAMD) scheme, in which a user can choose to communicate with the base station either directly or using relay transmission. We show that the RAMD scheme performs significantly better than Qualcomm's HDR scheme. Next, we propose a traffic-Aided "smooth" admission control (SAC) scheme that guarantees the quality-of-service (QoS) provisioning. Specifically, building on the weighted proportional fair (WPF) scheduling, we devise a QoS driven weight adaptation algorithm. The weights assigned to new users are increased in a guarded manner. An admission decision is made based on the measured throughput within a time-out window. Our results show that the SAC scheme works well in opportunistic communication systems.