I thought swiss ball crunches were a joke until I actually started doing them right. Reading Lyle McDonald's thoughts about ab training helped. He points out that your abs need slight spinal extension in order to contract fully. Regular flat-back situps from the floor are good up to a point, but they rely too much on your hips helping to pull you up when the weight gets heavy. I'm not advocating exclusively hitting the abs isolation/bodybuilder style but it helps to be able to work them directly when they're lagging behind.