Don’t work your abdominal muscles every day. Your abs are like any other muscle in your body – you shouldn’t work them everyday. You still need to give your abs time to recover.
You should train your abs 3 to 4 times a week, not on consecutive days.
For thicker, denser abs, you should train with added resistance. Just like your other muscles, when you train in the 8 to 12 rep range you are training for size and when you train in the 1 to 5 rep range you are training for strength. While there is really no need to train abs with anything less than 5 reps, by staying in the 5 to 7 rep range you will build strength and density in your abdominal muscles.
Use cables, dumbbells, barbells, chains, plates, medicine balls, a partner, or anything else you can find to add resistance to your crunches, twists, fold ups, etc…
For more abdominal exercises, read about the Top 10 Best Abdominal Exercises.
For smaller, thinner abs you should train with body weight only. By using lighter reps and 20+ reps you are training your abs for endurance. They will still look great once you lose fat, but they will be much smaller and less bulky. This is more ideal for regular women who just want to have super flat unimposing abdominals.
From time to time, no matter what your goal, you should still vary your rep ranges. Even if you want thinner abs you can still train heavy once in a while just to shock and stimulate the abs for growth. Similarly, if you want thicker denser abs you should still occasionally do some body weight exercises for 20, 50, even 100 reps. It is the variation in resistance that will train all the abdominal muscle types, which will inevitably lead to better abs in the long run.
To make your abs more visible you will have to lose fat. Decrease daily calorie intake and increase high intensity cardio to lose fat. Use endurance cardio as a last resort if the high intensity cardio is just too much on your body when combined with your strength training routine.