There are many things you need to consider when you are trying to develop and show your abs. This has been one of the biggest areas I have struggled with up until the last year. They will come with time and discipline, but they will come, if done right.
1.) The first thing you must consider is that the actual rectus abdominis will not be seen unless you get rid of the fat on top. Doing ab workouts alone will not do this. You CANNOT spot reduce. You must lower your total body fat and hope it comes off of the places you would like. The only tricks with trying to target belly fat all involve food. The biggest one is replacing saturated fat with unsaturated. Saturated fat is found in animals and is solid at room temp. This would be associated in red meats, poultry, whole milk, etc. The unsaturated fat or "good fat” is from plants and liquid at room temp. The best kinds are olive oil and fish rich with omega-3 fats such as salmon, tilapia, tuna, etc. There are many other benefits to switching these two kinds of fats out but I will cover that in a different note.
2.) The second thing you must do is build up the rectus abdominis. There will be no true 6 pack unless the abdominal muscles are pressing out against the tendons enough. I also want to mention that the "6 pack" is not actually 6 different muscles. It is one muscle separated by tendons as I just mentioned. There are different ways to train abs depending on the different results you want. Typically, when you want a slimmer look and a smaller waistline but still toned, you would want to do higher reps of failure. For most guys that want not just a tight core but protruding abdominals, I would recommend adding some weight doing less than 20 reps to failure.
3.) The third thing that you must make sure to remember is to train every area of your core. This means hitting oblique’s and serratus anterior just as hard as rectus abdominis. This type of training is mainly done by functional training of cross motions and isometric workouts.
4.) Do lower abs first. I hear more people say they struggle with lower abs more than anything. This is the case for me as well. I have found out that the biggest reason for this is because by the time I try to work them, I am already too tired from the other workouts which mainly target the upper portion. Try lower abs first since they are harder to activate. Every lower ab workout is also involving the legs. In order to develop this section you must be involving your legs to some degree. This can be anything from hanging leg raises on the captain’s chair to leg lifts. I will mention to be careful of working hip flexors instead though. This is hard to get around when working lower abs but you can get around it. A big hint is not to keep your feet in a locked position. Having your feet or ankles locked into place will cause your hip flexors to become activated. So instead of moving your upper body to your feet locked into place, move your feet and legs to your upper body. The last tip for these lower ab workouts is to keep your lower back flat. If you can’t keep your back pressed down flat into the floor then it will be taking a lot of the pressure away from your abs. If this happens and you are unable to correct it then get some more leverage by keeping your feet closer to your body. The second your back starts to arc stop the workout and reposition.
Hope this helps!