Let a killing frost blacken all your foliage. Cut off the main stalks leaving about a 6-8 inch "handle." Some people protect the open end of the stalk from rain at this point with foil or such. I use milk cartons and what a sight that is! You should transfer a label to the stalk or a stem so you know what variety you're dealing with- alot of them look exactly alike when out of the ground!

Wait a couple days then, using a tined fork or a carefully operated shovel, loosen the soil around 3 sides of the clump. I aim for about 8 or so inches out from all sides of the stalk and about a foot deep- often they are much shallower. Then you can get under the clump and gently rock it out of the ground; sometimes using the handle stalk to help lift it. Don't know if that will hurt your perennials or not, but they should be able to be patted down in place again, no sweat.

Once out of the ground, you can gently work off the big soil bits, rocks, worms etc, then wash it all well with a hose. Some people swirl the clump in a final wash of 10% bleach water, I save that for the individual tubers because there is invariably more dirt in tiny crevices you can't get to when they're still clumped. And a worm or two. At this point I jot down the variety on a couple good tuber candidates in case the label comes off. There are good pencils available that write on damp tubers without hurting them. No Blot Ink Pencil is one. Some use Sharpies on a drier tuber.

Let your washed clump dry a few hours, or overnight in a shaded, dry, non freezing place. Then cut off tubers (see link,) to prepare for your storage method. I use gallon zip lock bags with a little vermiculite inside to let the tubers dry a bit more before final storage. The bags are labelled for each variety.

If you read back to October/November of 04 you will find no end of threads on digging, dividing, storing tubers. I am a recent convert to the Saran Wrap method and will do that again this year for its excellent results.

If you do a search of dahlia websites available, almost all of them have advice on this process and a Midwest one might have more specific pointers for your zone.

Here is a link that might be useful: digging and storing