Almost any soil will benefit from improvement, and the overall health of the garden both over the long term and the short will be greatly improved by having a good soil to grow from. As a rule, the simple answer to soil improvement is to incorporate composts. However, the soil may need particular improvements, as indicated in a soil test. The goal is to create a soil with good tilth, appropriate levels of available nutrients, a desirable pH, and in most cases good drainage.
General soil improvement Edit
Improving soil in a general sense (with or without a soil test) is a physically demanding yet crucial task in the garden. As an old saying goes, "a ten dollar plant in a twenty dollar hole will grow much better than a twenty dollar plant in a ten dollar hole." That saying might be misleading in some ways (current thinking is that putting plants in "good holes" has similar long-term effects as would planting it in a container), but improving the entire garden bed will indeed make for great plants.
The best way to improve most any soil is to add composts, because a soil with lots of organic matter will be much more amenable for growing plants.