A large amount of research has been conducted to understand out how to fit running shoes. Most of the research that has been conducted has concluded that patient comfort is one of the most important variables.

Below is a great video that summarises the history surrounding this research, and provides practical tips on how to select a running shoe for you. To summarise:

choose a shoe that is most comfortable to you - do not base your choice solely on what a store person suggests;
alternate the shoes you regularly wear to reduce your risk of injury;
injury is multifactorial and appropriate runners are one component of a comprehensive treatment program.