Pick corn at the milk stage
The only way to really know if your sweet corn is ready to harvest is by pulling back part of the husk and checking the kernels. If milk spurts out of a kernel of sweet corn when you press it with your thumb, the corn is just right. If your fingernail punches into the kernel too easily, the corn is a little green yet. If you must press pretty hard to penetrate the kernel, it is too old. Older ears can be left on the stalk to dry for cornmeal; for eating fresh and freezing, though, you will probably want to pick at the milk stage.
Field corn can be left to dry on the stalk until late in the fall, harvested by hand, and stored in corncribs over winter. You won’t even have to bother shelling your corn before feeding in many cases. When the stalk is dead and brown, walk down the rows and pull off the ears, husking them and tossing them into a wagon or pickup truck alongside the row.
Husking is a skill you will develop; husking pegs, once made from wood or bone, are still available from some hardware stores and through the mail, and will help you strip the husk from the ear. Once husked, the corn should be stored in a crib to dry completely. Stalks left in the field should be disked under for organic matter after shredding with a shredder or even a rotary mower.
Bundling and shocking corn
Another method of harvesting corn by hand is cutting the whole stalk, not just the ears, and arranging them in bundles, and the bundles into shocks. To do this, you must use a corn knife and cut the stalks off with short downward strokes, leaving about four inches of the stalk in the ground. Continue down a row, gathering the stalks in your left arm. When your arm is full of stalks, drop them in a neat bundle. Later, you can tie the bundles with baler twine and shock them by leaning four of them together as if you were constructing a tepee. Arrange the rest around this central core.
The size of the shock is up to you; it might be best to tie several lengths of baler twine around the entire shock to keep it standing. Later in the fall when the pressing work is over and you have more time, haul the shocks in from the field and husk out the corn. You can store the corn in a crib or shed and feed the stalks and husks to cows, horses or sheep.
Excellent Popcorn can be harvested by removing the ears from the stalk but leaving the husks attached. The husks can be pulled back and used to tie several ears together, and these can be draped over a wire and hung from a rafter to dry. To keep mice from getting at the corn, poke a hole in a large tin can lid and slide it over the wire.
(Source – http://www.moongrow.com/vegetable_planting_guide/corn.html)