Managing Transaction Logs - http://www.sqlservercentral.com/articles/Administration/64582/

Why is my transaction log full - http://www.sqlservercentral.com/articles/Transaction+Logs/72488/

Stairway to Transaction Log Management - http://www.sqlservercentral.com/stairway/73776/

Some good links above on transaction log management.

As you have run shrinkdatabase I would strongly recommend checking for index fragmentation and rebuilding any indexes which need rebuilding.

As for the 50GB log file, what caused the log to get that big? One off data import? Poor transaction log management? Day to day activity?

Also remember that the log can only be shrunk to the most active point as the log is cyclical, so if the active portion of the log is marked at 49.5GB and you shrink it, it can only shrink it to 49.5GB, you will need to wait till the active portion of the log has looped round to the beginning of the file before you can shrink it down fully.