Recommended research articles or best practices for UI techniques for encouraging users to explore more functions of a complex application?

I can highly recommend the recent article by Cockburn et al. “Supporting Novice to Expert Transitions in User Interfaces” (ACM Computing Surveys, Vol. 47, No. 2, Article 31, Publication date: November 2014) but the research it reviews is almost exclusively about point-and-click interfaces that are trying to make small improvements in user performance.

What if you are trying to get users to be motivated to learn a new set of functionality in the app that they didn’t even know they would want?

As well as general research in this area, I’d also like to know if there is research or best practice about when to explicitly encourage the user to try something new.

When they open the app (like a Tip of the Day?) or just as they quit? If so, every time they open or quit the app, or spaced out, or randomly?