Be sure it is worth it.

Smart developers can smell an arbitrary deadline a mile away. The need for extra effort need to be tied to something more tangible than the Gregorian calendar and/or an executive's bonus.

Stimulate a culture of ownership.

Interest your developers in the competition. Your competition are fools and trolls. Certainly we can do better. Regularly checkpoint against the competition.

Consider developer-suggested features. Developers will put in the extra effort if they are playing out their own ideas, and are motivating each other.

Consider the following scenarios: Manager: "I need you to work this weekend to get xyz done by mmddyy."

Or, Developer: "Dude, I am going to hackathon on xyz at the coffeehouse on Saturday. Caffeinated mints are on me."