SQL Server How to Set up Maintenance plan to shrink log
When you switch the recovery mode to shrink the log file you are invalidating prior backups because it breaks the log chain.  What you need to do it switch it to simple, shrink it down to what you want, then switch it back and set up regular log backups (I would recommend hourly) in addition to full backups.  When a log backup is performed the log is truncated (note, not shrunk.  it will remain the same size on disk) and then the database will be free to re-use the space at the beginning of the file instead of appending to the end of the log file and causing file growth.