So I'm banging away on Book 2 now. The main adversary of the protagonist is hinted at in Book 1, so I'll give you a clue to look for if you read it.
About midway through the book, I spend a little extra time having the characters dwell on something that's irrelevant to the main plot. It's subtle and innocuous, but a sharp reader would go, "Why's the author spending time on this thing right now?"
Well, naturally, the reason is to foreshadow events in Book 2. Whenever I've read a book series, I've always found the most enjoyment in realizing that one book hints at events in the next book. For whatever reason, that gives my reader-sense the tingles.
So, obviously, I'll be doing a lot of that in my series. I did my best to cut out anything not essential to the story arc as a whole, so what you see between the covers is all relevant to where the characters are going.
Anyway, I hope these seeds prove as enjoyable for my readers as they were for me. I guess we'll find out one way or the other!