(Exercise)
Treat abs like any other muscle group you were trying to make gains on. Maximize the tension they're under and abandon the high repetition stuff. Think of more taxing exercises such as the following:
Windshield wipers on the pull up bar
L sits on the dip station (static isometric hold)
Hanging leg raises on the pull up bar
Dragon flags on the bench (static isometric hold)
Experiment with your weight, and discover the range where your abs are more easily visible. For example, at 170lbs and under my abs really pop. If I get over 180lbs I start to look bulkier, stronger but not as defined. I've experienced this difference in appearance several times. You may have a weight where you'll have the most definition, or like your look the best.
Of course cardio is a big one. HIIT, and roadwork (running) both have their place. HIIT training makes me look more vascular, but running slims my waistline so I use both.
Mix it up, and experiment.
(Nutrition)
If that doesn't work you could count your calories, and lower them week by week as you try to get to your ideal look. Remember to take pictures. We sometimes can't appreciate the progress we've made if we are recalling in our minds what we use to look like. Get a collection of photos, and track the exercises you were doing and the foods you were eating when each was taken.
Drink plenty of water. It flushes toxins, and skeletal muscles are about 70% water. Water can also act as an appetite suppressant. Research macro, and micronutrients as well. There is a wealth of knowledge, and studies done to see how they affect the human body that should be available online. If that doesn't work, there may be a nutritionist in your area that you can consult. They'll have the most science based recommendations for what you should be eating.