diet diet diet diet diet diet diet.
diet. Abs are made in the kitchen!
The reason programs like P90X (and Starting Strength, the official internet fanboy routine, which I do support!) are so effective is because they're __comprehensive__. Most people need a program that covers all the loose ends for them, particularly the nutrition. Going on a carrots only diet doesn't really work.
When your nutrition ties directly to your fitness, assuming you're rigorous with both, you really see the benefits. I agree that 'the best workout/diet is the one you stick to' to some extent, but there's certainly wiggle room there. Weight lifting on a linear progression program is extremely rewarding and gives some of that instant gratification people want from exercise. Having taken a few friends from couch-to-starting strength, the joy on their face the first time they squat 45s on each side is always the same.
In addition to the diet, you have to overcome the "but I'm not Ahnold!" Nobody starts out like the Governator. Getting a buddy to eat that piece of humble pie and going into the gym benching only the bar can be damn near impossible.