For a 6 pack...just don't be fat. Reduce bodyfat under 10% and you should see your upper abs. Under 8% and you may see the middle line. Under 6% and you'll have lower abs with veins.
As far as exercises than work the abs....deadlifts and squats. Studies have shown those two exercises activate the abdominal muscles more so than "crunches". The abdominals are not "designed" to just bend you in half. They are designed moreso to resist motion and to remain contracted for long periods of time. So pushups, planks, etc will also work.
Crunches/situps activate the hip flexors moreso than the abdominals.
Alan