In some cases, parties may sign a "non-waiver" contract which specifies that no rights are waived, particularly if a person's actions may suggest that rights are being waived. This is particularly common in insurance. Sometimes the elements of "voluntary" and "known" are established by a legal fiction. In this case, one is presumed to know one's rights and that those rights are voluntarily relinquished if not asserted at the time.
The following represent a general overview of considerations; specifics may vary dramatically depending on the jurisdiction.