Select one or two of the exercises from exercises for the abdominals
. You don't really need more than one or two and you don't need to do them more than two or three times a week at most. If you do you risk hurting yourself more than you benefit yourself.
Next comes the very true adage of "Six packs are not made in the gym, they are made in the kitchen".
To be perfectly honest, your muscles barely need any definition to look like the abs of those people with "washboard abs". They simply have almost no body fat. To get a six pack, you need to reduce your body fat to around 8% if you are male, or 14% if you are female. The way to do this is with regular aerobic exercise and a diet wherein you burn more calories than you take in. Eating a large amount of protein, say 40%, is recommended.
The reason the average USAmerican citizen isn't going to be appearing on the cover of a health magazine is because of their diet.
A caveat on gimmick exercise machines: Some work, most don't. If they really do follow the rules in terms of exercising the muscle groups they claim to, then absolutely they'll work, but almost none do this. Regardless of what those machines do, none of them top simple crunches, and nothing tops simple crunches like a combination of simple ab exercises and a healthy diet. The abdominals are more like an accordian than a hinge. Bearing that in mind, don't trust exercise machines that treat them like a hinge.
Of course, to all the zen masters out there, this node is worthless, since they are at peace with themselves and don't care to change their appearance.