There are a few things to remember when striving to get the abs you desire.
1. Genetics - Everybody has "abs" so the question shouldn't be 'how do I get abs.' It should be, 'how do I get to see my abs.' The thing is, they cannot be seen on everybody because on most people there is a layer of fat cells covering them. When you can finally see them, remember that everybody has a different abdominal structure. No two sets of abdominal muscles look exactly the same. Some people have lopsided abs, some have big block abs, and others have many smaller abs. No matter what type of abs you have, they will look impressive if you develop them to their fullest.
2. Diet - You don't need to diet to get abs, however, you DO need to diet to see them. It takes a lot of hard work and willpower. If it were easy to have a good defined set of abs, everybody would have them. It is imperative that not only do you train your abdominals, but restrict yourself to a diet that will allow you to lose body fat. Only then will you begin to see your hard work as the fat dissipates and your abs begin to shine through. Abdominal work is only 10 percent of the ab struggle. The dieting aspect is 90 percent and without it, you will never bring your abs to their fullest potential. If you don't have the will to diet, then you don't have the will to have the nicest midsection that you can possibly possess.
3. Rep Scheme - You need not do "a hundred crunches every night" to get the abs you want. You may hear people that boast that they do "100 crunches every night before bed" or 500 sit-ups a day. I am telling you right now that this is by no means necessary not to mention usually an enormous waste of time. Treat your abs how they want to be treated. If you take care of them, then they will take care of you. By taking care of them, I mean training them as if they were any other muscle when first starting out. This means you should be doing no more than 20 repetitions at a single time. Any more than this and you are just building muscular endurance. To have a showy midsection you do not need endurance you need abs! If you can do more than 20 sit-ups, chances are that you are doing them too fast anyway. This is not to say that you can not do hundreds of sit-ups, crunches, leg raises, or anything else if you want, but it is to say that the latter is not what your workouts should consistently consist of. If need be (for shocking purposes or otherwise) do your 100 sit-ups before bed if it will make you happy, but don't let it take over your normal training. Use it for variation in your ab session only.
4. Variety - You are the army and your abs are the enemy. Attack your abs from every angle and you will have the advantage over them. Sit-ups and crunches are great, but you must remember that they are targeting the same areas of your overall midsection. To have a complete and striking set of abs, you want them to be ripped from intercostal to intercostal. The only way you can accomplish this is by not neglecting any one part of the midsection. The intercostals and external obliques (side abdominals) are what are going to make you stand out from the rest of the crowd. All you have to do is throw some exercises for them into your abdominal routine and you will look incredible!
5. Proper Use of Exercises - To get the most out of every workout, it is critical that you know exactly how your movements affect your muscles. For example, when doing a crunch you are not lifting your lower back off the floor. This is isolating the upper portion of your rectus abdominus. As you start to bring your lower back off the floor, your abs are still being hit but you begin to use your hip flexors to help move you up. This is what constitutes a sit up. Know what you are doing when you are doing it. Don't just go through movements for the sake of going through them. Feel every single inch of them. Know what's going on inside you and you will be well on your way to seeing the results you want and deserve!
One last note: PLEASE do not fall for any exercise contraptions that you see for sale on TV in the middle of the night. They are overrated, overpriced, and overused. They are NOT necessary to make any gains whatsoever. They are simply sold for someone to make money and they are NOT as beneficial as they claim to be. No machine in the world can take the place of good old fashion abdominal movements such as crunches, sit ups and leg lifts. Don't waste your time, don't waste your money, and don't waste your abs on these wastes of space.
Good luck in your training efforts. Train hard and never give up!