Check out stuff by Stuart Mcgill. Here are a few links you might find helpfull.
HalfSquare: Caught Between Being Cool and Being Square This gives a very brief look at McGills "big three" Do more research on him.
Better Golf With Fitness: No More Sit Ups and Crunches
GolfWRX.com > No More Sit Ups and Crunches, Part 2 these are good ones from Anthony Renna on how his core training has evolved.
I also like the chop and lift per Gray Cook. The articles discussing that can be found on his site functionalmovement.com under the articles or in his book athletic body in balance.
Hope this helps.
Also... the idea of bracing seems better than "drawing in" or belly button to the spine. Read McGill for more info on this.