Actually, as Richard has often said elsewhere, the key comes long before testing;
In fact, the key comes before you even write the first line of code;

The key is in a full & through design, and then in a rigorous implementation process.
Be sure that everything is throroughly reviewed at all stages throughout the process.

As the old QA saying goes, "you cannot test quality into a product"

I think in this respect another saying is even more applicable

Succesful testing does not prove the abscence of bugs, it only proves the abscence of known bugs

Erik