SQL Server Reporting services migration (2008 to 2012)
Best is to use  Reporting Services Scripter


  Reporting Services Scripter is a .NET Windows Forms application that enables the scripting and transfer of groups of SSRS catalog items from one server to another or from one SSRS folder to another on the same server. Reporting Services Scripter is flexible and lets you transfer catalog item properties, such as serverside report parameters, Subscriptions (both normal and data-driven), Descriptions, History options, and Execution options (including both report-specific schedule and shared schedule execution options) to other SQL Server instances.


Kevin Kline highlights the functionality here and there is a good article by Dale Kelly for migrating reports using RSScripter at MSSQLTips.

Another way is to use the power of PowerShell. There are many script available like here and here.

There is other utility (which I have not used) called reportsync