Fix your nutrition. Abs are made in the kitchen.
Cut out processed foods. Don't care if you're picky; eat good food. Start eating fruits and vegetables, meat, fish, etc. You need a caloric deficit to lose the fat covering them.
For abs work, RKC planks, L-sits, hanging leg raises all good options. Don't do situps or crap exercises like that. Back flips are OK if you can do them safely
Stick to the outside lap of the Grocery store and shrink your Wheat based carb intake (Breads, Pastas, And BEER [that's the hardest for me] etc.)
Heavy Lifts and Fast Paced W/O's keep your body burning for hours after. Sprint Training and Multi Joint Lifts (Olympic Lifts) should be your bread and butter.
Add in the Ab work Steven said to give your abs a nice long look once they are visible. What I'm saying is, Just because you are working your abs and they burn, doesn't mean you are burning the fat around them.
I like Windshield Wipers, Leg Raises, Russian Twists, V-Sits and Holding Cat Grabs is a pretty sweet Core Workout.
What steve said + normal work schedule. This means go to bed at the same time and eat at the same time almost everyday. f#cking up your sleep can make you look a little flabby sometimes.
Oh, and Keep track of Sodium intake. To much will cause you to retain water and hide those abs. Can be countered by drinking MORE water, this will also help you burn fat.
Not drinking enough water is VARY often overlooked here because it seems like everyone should already know, but it's easy to think "I get enough water, I drink like 8 glasses a day" "Just like people who say "I'm skinny and I eat a lot" then finding out they only eat about 1900-2100 cal. Athletes should be getting a gallon or more a day.
Hope this helps. Got me motivated to get ready for beach season. Just kidding (Not really)
Post is not Medical or Professional advice. Just opinion based on my own experience