I played baseball for 18 years, and in college, abs was always the last part of our conditioning, everyday, for about 10-15 minutes. We would do a mix of about 10 to 20 different exercises daily. Some of them were - feet on the floor crunches, 90degree crunches, legs straight in the air ankle touches, feet on the floor ankle touches, feet on the floor alternating ankle touches, feet on the floor opposite ankle touches, V ups (legs up and arms up at the same time) , bicycles (hands behind head, side to side, opposite leg kick in at the same time), russian twists (side to side touches with both hands), leg lifts, kick ins, one leg kick ins, stabilizers (body weight on your elbows and toes, balance with your back straight)
for my own workouts now, I usually do 3-5 sets of 3 or 4 of those, everyday