With the emergence of grid technologies, the problem of scheduling tasks in heterogeneous systems has been arousing attention. In this paper, we present two optimization schemes, Makespan Optimization Rescheduling (MOR) and Resource Optimization Rescheduling (ROR), which are based on the QoS Min-Min scheduling technique, for reducing the makespan of a schedule and the need of total resource amount. The main idea of the proposed techniques is to reduce overall execution time without increasing resource need; or reduce resource need without increasing overall execution time. To evaluate the effectiveness of the proposed techniques, we have implemented both techniques along with the QoS Min-Min scheduling algorithm. The experimental results show that the MOR and ROR optimization schemes provide noticeable improvements.