I think ab training is inconsequential from an aesthetic point of view, I think muscularity and vascularity in the abdominal area is achieved entirely through good, sound dieting techniques, but that's just me. I mean, I went off abdominal training altogether for 8 weeks, and retained my abs from a clean diet and heavy squats/deadlifts. Now I've gone back on solely because I believe in the merits of a solid core, which has lead to higher poundages in all my core-based lifts. I recommend a 5x5 routine, composed of cable crunches, incline Russian twists and dragon flags/weighted knee raises. You can alternate these with weighted Swiss ball crunches, ab roller crunches, abdominal bridges, hanging leg raises, Swiss ball rolls, etc. As far as oblique strength goes I like one-armed deadlifts and twisting cable crunches. One exercise that DOES help with abdominal separation, though, I must say, are vacuums.
Peace.