SOIL
Dahlias like loose, fertile soil, around 60ºF and not bone dry but not at all soggy. Set yourself up for success by doing a soils test - this will help you understand what your particular soil needs. I use UW-Madison's Soils Lab. If you are unable to get a soils test, mixing a bit of compost into your soil is a great way to add nutrients and improve drainage. I also throw a bit of starter fertilizer with mycorrhizae into my planting holes.
SUNSHINE
Dahlias require full sun, meaning ~8 hours of direct sunlight. More sunlight makes for stronger and sturdier plants with more blooms.
There are a few varieties that *don't* enjoy full sun in the heat of summer. We've made notes for those varieties - they are best planted with some shade cloth, or in a spot that gets a little shade in the hot afternoon.
WATER
DO NOT WATER dahlia tubers at planting time! Tubers contain all of the moisture they need to start growing, and are quite likely to rot if watered too early. Be patient, and wait until you see green growth above the soil - then you can water them. :) It can be a few days, or a few weeks! Once they are actively growing, dahlias appreciate a deep watering a few times per week, and more frequently in the hottest parts of summer.
PLANTING & STAKING
Plant your dahlia tubers horizontally (vertically encourages rot), ~5" deep and at least 18" apart. I recommend adding a 3' stake at planting time, 8" deep and 6" from your tuber crown. Happy dahlias get tall and heavy with blooms, and staking them keeps them growing straight ands strong. Staking them early eliminates the risk of stabbing your growing tubers later beneath the soil! If you are growing a lot of dahlias, there are more efficient ways to support them, but staking works very well for under 20 plants. :)
LABELING
This was a mistake I made in my early growing years - not labeling correctly or often enough. If you want to keep track of what you are growing (And you really should! Especially if you ever plan to share or trade tubers!) it is crucial to label your tubers 3 times per season: when you plant them, after they've bloomed, and when you dig them up.
When you plant them: My first label markers go in the moment after I lay a tuber in it's planting hole, and I place them 12" in front of the tuber crown. I use a plastic or metal stake labeled with a garden marker. Don't use a sharpie - it will fade in the sun almost immediately. Don't use wooden stakes - they'll rot and crumble.
After they've bloomed: I always do a follow-up ID to confirm whether the bloom was true to the variety I thought it would be. I like to label a bit of flagging tape and tie it around the base of my plant. That way when it comes time to dig, I know what I've got.
When you dig them up: Make sure your labeled flagging tape is connected tightly to the stalk that stays connected to your tuber clump- if there's any risk of it falling off, I also tie a label through the clump, or write directly on a tuber with a sharpie. Digging/dividing is the biggest danger zone for tuber identity issues!