When You Recieve a Tuber Shipment
Examine the tubers. If they are mushy, ask for replacements. If they are firm and smooth skinned, store them as is in a small amount of vermiculite or wood chips near 40 degrees F with ventillation until planting time. If they have wrinkled skin or slightly soft, they are probably OK but may need a little moisture added to chips (just slightly dampen) and then store as above.
Quick and Dirty
Bury the tubers 3"to 4" deep in spring when soil is warm. Water them thoroughly whenever soil dries. Cut flowers when they look good (~August). Dig, wash, dry and store tubers in fall if your soil freezes. Split and mark them.
The more Detailed Version
In spring after all danger of soil freezing is past:
Spade up soil one foot deep. Fill bottom of hole with decayed mulch. Optionally, mix in some fertilizer (I use 10-10-10 or similar). Place tuber 3" to 4" below surface with eye (bud or shoot) up. Drive a 3' stake 2" from eye end of tuber. Put a tag on the stake to identify the variety. Cover with soil.
Care and feeding:
Dahlias need about one inch of water a week. It is best to water them once a week if needed and let the soil drain until the next watering. A side dressing of 10-10-10 or similar once or twice during the season will make the plants more vigorous. Tie the stalks loosely to the stake as they grow to prevent wind damage. Spray foliage with Malathion or Diazinon (alternately) every two weeks if you want to limit bug damage.
For Optimum Cut Flowers:
Pick off the buds on the sides of the shoots leaving only the lead bud to get long, straight stems. Cut flowers during cool part of day and immediately place in water in the shade. Change water and cut an inch off stem on a diagonal daily. You can also soak newly cut stems in 98 degree F water for 30 minutes or so and add a feeding solution to the water. Pick off dead flowers that you don't cut to leave energy for new flowers.
In Fall after stalks have frozen:
Cut stalks a few inches above ground. Dig carefully all around to a spade depth 8" or more from plant. Use spade as a lever and stalks as a handle to lift tuber clump from ground. Thoroughly wash the dirt from the clump and turn it upside down to dry. You can divide clumps into individual tubers in fall or in spring. You need at least one bud on each tuber. Store them so they won't shrivel much (not too dry) with some ventillation, between 40 and 50 degrees.
Some gardening books provide more details. A book specifically about Dahlias is a must for a serious grower.
The BEST source for Dahlia information is the American Dahlia Society which has great publications and peer groups in most areas. Membership inquiries can be sent to: Alan Fisher, ADS Membership, 1 Rock Falls Ct., Rockville, MD 20854.