"Till in lots of compost. I like to apply a thick (6 inches) layer of compost, then mix it really well with a tiller. The trick is to get the soil just at the right moisture level when you till -- Barely moist, so that clod is crumbly, not hard and not squishy like putty.
An alternative, if you have no tiller, is to make a lasagna garden. Lay down a solid sheet of cardboard or newspaper, then layer brown garden waste (leaves, etc) alternating with green waste (grass clippings, kitchen waste, etc). Do this until it's about 2 feet high. It will decompose and settle back down in several weeks.
I just like to till in compost -- I think it ultimately provides a better, deeper garden soil. But I know of many gardeners who are quite happy with their ""lasagna"" garden. "