This paper addresses the problem of excessive memory usage in simulators using asynchronous conservative parallel discrete event simulation algorithms. The characteristics of the modeled system that promote this problem are examined.Existing algorithms that attempt to solve this problem are discussed and new flow control algorithms introduced. Results of simulation experiments using both existing and new algorithms are presented.The results show that some existing algorithms fail to achieve their goal. They also show that all of the existing algorithms are sensitive to parameter settings. The new algorithms are shown to be effective and less sensitive to parameter settings.