Six pack abs - a the sensible approach
Apparently everybody wants abs. Well here's the good news: you already have them! Unfortunately, they're probably under-developed, and hidden under inches of fat. But at least you have them, right? Now you probably don't just want abs, you want well developed, visible abs, as if just having them isn't good enough anymore.
Simply put then, there are two things you need to do:
1. Lose the fat
This is the harder of the two parts, unless you're already extremely thin. If you pinch your stomach and all you can grab is skin, you can skip this section. But don't be too smug, because if you're so thin and don't already have good abs it's because you have no muscle mass whatsoever. Get down to number 2, and find out how to put some muscle over those ribs, before you poke someone's eye out.
Now if you do have fat to lose, here's the bad news: you have to lose almost all of it before you'll get good ab definition. Take for example Magnus Samuelsson, former World's Strongest Man. You can't tell me he doesn't have strong, well developed abs, but as you can see it doesn't take too much fat to cover up any definition. What's more, abdominal exercises (crunches, situps, etc) don't target the fat on your stomach. When you lose fat, you lose it from all over your body, not just from near the muscles you're using. That's why those ab machines don't do what they claim, otherwise you'd see fat people walking around with huge, flabby arms and legs and ripped six packs.
So what's the answer? Lose the fat with a good diet, and a well rounded exercise program. That means a mixture of cardio and resistance training, but if anything the diet is more important. Short answer: ensure you're getting one gram of protein per pound of bodyweight per day, and reduce your sugary or starchy carbs (junk food, grains, bread, rice, etc). Long answer: read more specific articles on dieting and fat loss.
2. Train the core
Now you just need to train your core to make sure that, once the fat is stripped off, you've got something underneath that actually looks good. While you're at it, why not strengthen your core so it can actually hold your spine in place like it's supposed to? If you're lucky, you might even avoid becoming one of the many (many) (many) people suffering from back pain. It would be a shame to have to show those abs off from a wheelchair, wouldn't it?
Before I tell you what to do, I'll tell you what NOT to do: hundreds of crunches per day. Sure, you'll get good at doing crunches, but more likely than not you'll develop a sizable back hunch before you get good looking abs. Flexing your spine forwards hundreds of times a day, especially when you probably use your hands to pull your neck forward too, is not good for your back. It's not good for anything, really.
Basically, your spine can move in 3 planes: front to back, side to side, and rotation (I'll spare you the technical terms because I know you don't care). Your core therefore needs to be able to resist those movements, and so a good basic core strengthening program should cover all three. Include the exercises below once or twice a week with whatever other exercise you do, and maybe you'll even turn your jelly back into something respectable, and you can send me all the money you save on back pain treatments.
The abdominal plank
Rest all your weight on your forearms and toes, and hold that position for time. Do a few sets of as long as you can do. To give you an idea of how long to hold it, if you can't do 30 seconds with good form, you really need some work. If you can do 2 minutes easily, then congratulations, you won. Or you're not doing it right.
It's important that you maintain a straight-ish line with your body. Some people dip and rest their gut on the ground, bending their back in the process. Others stick their ass in the air. Both are bad. You should also keep your hips tucked under, or posteriorly tilted. Don't anteriorly tilt your pelvis, because that's bad too.
Now for the OTHER side of your body, you have to train the lower back and the glutes. If you think about your core like a building, this is for the opposite wall. Without it, the building would fall down. Ready for a really creative name? It's the glute bridge.
Do this one for a few sets of 10-20 reps, focusing on squeezing your glutes and getting your hips as high as possible. There's less room for you to stuff this one up, so no real warnings from me. Either do it with both feet on the floor, or if that's easy, lift one leg straight up and push with your other leg. Try to touch the raised leg on the ceiling.
Remember the abdominal bridge? This is the same, except for your sides. If you don't care about having four walls to the building that is your core, maybe you care that this will train your "side abs". For a while after the movie came out every second exercise question on the internet was "how do I look like Brad Pitt in Fight Club?". Unfortunately I have no exercises to make your face like Brad Pitt's, but that's why you want good abs, right? Creative name for this exercise: side bridge. Support your weight on one forearm and the sides of your feet, and point your other arm at the ceiling.
Make sure your body makes a straight line from feet to shoulders, again don't let your hips sag or lift too high. Also don't drift too far forward or backwards, you want your body to be as straight as possible. Train this exercise with timed holds, as with the abdominal bridge. Make sure to change sides as well, or you'll develop muscular imbalance and maybe even a nice curve in your spine if you didn't get one already in the gene pool lottery.
Don't hold a plate and swing it side to side like you see people in gyms doing, and please don't put a broomstick on your back and twist back and forth like an idiot. This exercise requires some form of horizontal resistance in the way of a cable machine, rubber tubing or stretch band. It's like a Pallof press except you don't move your arms, so let's call it a Pallof hold. Simply hold onto your cable/band with both hands outstretched in front of you, stand side on to wherever your cable/band is attached, and hold it there. There should be enough tension that it is trying to pull your arms towards the attachment, and you have to work to keep it steady. And that's about it – easy as anything. Do timed holds as with before – a few sets of 30 seconds each side will do it.
And there you have it. Those four exercises will make sure your core is doing what it's supposed to, and you'll develop the muscles while you're at it. Combine them with some serious dieting and you'll be seeing your abs in no time.