Baruch HaMavdil Bein Kodesh L'Chol To Do Melacha

After it is dark, say Baruch ha'mavdil bein kodesh l'chol (just those words--not God's name or any of the standard words used in blessings!) if you want to end Shabbat and do weekday activities (melachot) before saying ma'ariv's amida or havdala.
Reason This fulfills the commandment of “zachor” for Shabbat and allows you to do melacha
Note But it does not allow you to eat or drink, once you have finished se'uda shlishit, until you hear or say havdala.