well you wait for the first hard frost, that turns the plants into looking like cooked spinach (as another member aptly described it). take some heavy duty cutters and cut them so the stalks are only about 6"above the ground. Some people like to cover the tops of the stalks with aluminum foil to keep moisture from going down the hollow in the center of the stalk and rotting the tuber. This is because you will then let them sit in the garden for a few days, up to a week I think. This time forces the goodies to go down into the tuber and 'feed' it as it were. Then you dig them up gently with a pitchfork. Kind of go around them with the fork, about a foot from the stalks in either direction so your radius is a couple feet wide. It may seem like alot until you see the size of tuber masses on some dahlias.
Gently pry them up from the soil and set aside. If you have more than one type you may need to keep track somehow of which is which. If they are really muddy, hose them off. Let dry a bit. Then comes the experts on dividing tubers. Must find link. I think it is Jroot who posted the definitive work on separating tubers.
As for storage, there are many options. It really depends on your climate and what you have to store them in and its relative dampness. Some places and methods will cause them to shrivel and dry up while others will turn them to rotten mush. I will let others take over from here.

Happy reading. This is a long thread but extremely informative.

Here is a link that might be useful: diving link

Thanks. triple_b. The info is extremely helpful.
Some of the pics from "Bernie's" email in that link are gone.
Wonder if you have the post saved or the American Society of Dahlia's link.

what part of the American Dahlia Society are you looking for?

Here is a link that might be useful: American Dahlia Society

time to grab yourself a mug of Joe and get comfy in the computer chair.
Just make sure you compare the different storage methods. Different parts of the country will have different levels of humidity so what works in, say, Oregon, will not work for you.

Greenbug,

Do be patient about digging up your Dahlias, its really nice to see blooms even after the first frosts. Remember, you're tubers will be fine until the ground is frozen hard.

Cheers,
Russ

yeah, what he said. :-)