As Rodja mentioned only a good diet and low BF will allow the abs to show through but for a good routine you can do far worse that the P90X Ab Ripper X routine.
Each exercise is 25 reps and minimal rest (i.e. less than 10 secs) between each exercise:
In & Out
Seated Bicycle (25 forward and 25 reverse)
Seated Crunchy Frog
Crossed Leg/Wind Leg Sit-Up
Fifer Scissor
Hip Rock 'N' Raise
Pulse Up (Heels to Heaven)
Roll-Up/Pull-Up Combo
Oblique V-Up (25 each side)
Leg Climb
Mason Twist