What is the best and easiest way to turn off SQL Server when not in use?
You may need to access SQL Server Configuration from the Start menu by selecting Microsoft SQL Server and then Configuration Tools, and then follow Thomas' path.

You can also configure those services so that they don't start automatically. I have mine set on "Manual", so if I want to use the SQL Server server, I just start the service. Ditto for SSIS, SSAS, SSRS, etc. 

Since I have two versions of SQL Server running on my development machine, I don't want all of this stuff eating up my resources.