Topic #1: All things subtle.
Foreshadowing, hints. Subtle things to keep the readers on the track you want them to be on, until abruptly, the track ends, and suddenly they're on a different path--one they (hopefully) never saw existing.
I love subtle things, because it makes a story that much more interesting. It adds depth to the characters and plot, and if done correctly, can seriously make a story stand out.
However, what I hate about it is that it's very difficult to skillfully master. The best foreshadows are the ones we never see until it happens. Think: The Hunger Games. In the beginning, Katniss starts out reaching for Prim, only to come up empty-handed. In the ending of Mockingjay, Katniss tries to reach Prim, and-
Ends up empty-handed.
Subtle foreshadows such as these really awe me, mostly because it's already hard enough to keep up the hints in one book, much less a trilogy. Also, subtle things are what they are--subtle. Sometimes, when I go back to read a chapter, I find a thread of information there that could easily be a foreshadow, but I just never planned for it to be there. They say the best ideas come during the most unexpected times, and it's the same with things that are subtle. If we can let it glide with the text and suddenly turn it conspicuous at the end, something that will amplify the impact of the ending, then what you have achieved deserves a standing ovation--especially in a series.
However, being subtle is hard just like that, because how can we possibly make a single line blend in with everything else without making it out-of-place, or something a reader can suspect as a foreshadow? How can we remember all these subtle things we've put in our writing? How can we execute it just right so that the result is purely satisfying?
Candidly, I don't know. But what I do know is that you should always trust your guts. Read lots of books. Write a lot. Re-read your writing. A lot of times you'll catch a line that can conspire an unexpected twist--if it's there, maybe it's a sign. Or maybe it just sounded good there.
Who knows? But in the end, the only way we can create great subtle shifts or foreshadowing is by practice, practice, and more practice.
And a lot of chocolate.