Not How, But What

A happy developer is a productive developer. That fact should be obvious to everybody who interacts with a developer of any sort.

Developer happiness is affected by a lot of things. The work environment, the projects they are given, their managers, the flexibility of their schedules, and the quality of their tools. This last one is very important. If a developer doesn't like the tools they are using, it's a safe bet they won't be happy, and thus won't be as productive as they could be.