Process sessions of workflow.

i have a workflow triggering  on change of date time field . the workflow waits for reach of date ,then further process is done.

lets say for the first trigger process session is created and waiting for reach of date .and if before date is arrived if the user again changes date ,workflow is again triggered and another process session is created .

now i have two process sessions waiting .

i want only one to be waiting at once .how can i manage this .

Thanks,

Mohammed Waseem.