Don’t start too early. If the ground is still semi-frozen or soggy, digging in the soil can compact it and harm its structure. How do you tell whether it’s ready to be worked in? Grab a handful and squeeze — it should fall apart, not form a mud ball.
Add lots and lots of organic matter! Try using compost, dehydrated cow manure, shredded leaves, well-rotted horse manure (call nearby stables), or a mixture. If your yard happens to be blessed with fertile soil, adding organic matter is less crucial, but most soils can stand the improvement.
Allow the cultivated soil to sit for a couple of days before planting. You can turn the soil daily if you wish, but the process is not necessary if you turned the soil adequately already.