Cream cheese must be room temperature before starting or you will end up with a lumpy finished product. If using the same whisk to whip cream and mix the cream cheese and sugar, you MUST whip your cream first. Any oil from the cream cheese left on the whisk can cause your whipped cream not to whip up. Leftover parfaits will keep just fine overnight, but if making for guests or a party, they should probably be made shortly before serving because the cream will deflate slightly (you can see this with the tall Iron Man parfait in the top picture).