I love the prone jackknives. But they are hard! I'd rather do them than the swiss ball crunches, which are boring.

Have you seen "New Rules of Lifting for Abs" (which is in fact about core strength of all kinds) yet? It is a crunch-free zone! They've come down on the side of the research that suggests that crunches of any kind are more trouble than they're worth. \o/ \o/ \o/