Truncate all tables in the database. Source: http://stackoverflow.com/a/12719464/108374

TruncateAllTables.sql

exec sp_MSforeachtable 'ALTER TABLE ? NOCHECK CONSTRAINT ALL'  
exec sp_MSforeachtable 'ALTER TABLE ? DISABLE TRIGGER ALL'  
exec sp_MSforeachtable 'DELETE FROM ?'  
exec sp_MSforeachtable 'ALTER TABLE ? CHECK CONSTRAINT ALL'  
exec sp_MSforeachtable 'ALTER TABLE ? ENABLE TRIGGER ALL' 
exec sp_MSforeachtable 'IF NOT EXISTS (SELECT *
    FROM SYS.IDENTITY_COLUMNS
    JOIN SYS.TABLES ON SYS.IDENTITY_COLUMNS.Object_ID = SYS.TABLES.Object_ID
    WHERE SYS.TABLES.Object_ID = OBJECT_ID(''?'') AND SYS.IDENTITY_COLUMNS.Last_Value IS NULL)
    AND OBJECTPROPERTY(OBJECT_ID(''?''), ''TableHasIdentity'') = 1
    DBCC CHECKIDENT (''?'', RESEED, 0) WITH NO_INFOMSGS'