I use these 2 statements as two different steps in a job to back up the log and then truncate it.  Works well.

Step 1

BACKUP LOG [MyDB] TO  DISK= 'D:\Backup\MySqlServer\MyDB\MyDbLog.dat'  WITH  NOINIT , RETAINDAYS=2, NOUNLOAD ,  NAME = N'MyDB Translog Backup',  NOSKIP ,  STATS = 10,  NOFORMAT

Step 2

USE master

BACKUP LOG MyDB WITH NO_LOG

GO

USE MyDB dbcc shrinkfile('MyDB_log')

GO

Good luck,

Jeff