Whether your working with heavy clay or sandy soil, adding a lot of organic material is the trick to healthy, rich soil.  Organic matter helps to lighten clay soil and helps sandy soil hold moisture.  Organic matter should be added yearly.  Great soils get better every year.  In the spring, good sources of organic matter are soil pep (ground up bark), cotton bur compost, and your compost pile. 

In the fall, cover your garden with leaves and grass clippings.  Till them in, to break down over winter.  If using well rotted manure, add it only in the fall to prevent plant burn that can occur if it is applied in the spring.