I've always found that training my core for wrestling was best done in a more practical way, as in no crunches or anything easy like that. I read a quote about core strength for olympic lifting that said something like you are lifting standing up, so train your core for that. Following that, I would say to try awkward or harder moves for your core-
Make sure you focus on breathing, when you are doing a core movement breath out to tighten your core, and remember that when sparring as well, it will help.
I heartily accept the motto, "That government is best which governs least"; and I should like to see it acted up to more rapidly and systematically.