Are the log files continuing to grow? Or are they just not shrinking to a smaller size?
If the log files are continuing to grow in absolute size, you might have an open, uncommitted transaction (check by using SELECT @@TRANCOUNT) somewhere. You will need to find and kill that transaction.
If the log files are not growing (but aren't shrinking down to a size you are comfortable with), I would read a couple of topics in BOL:
Virtual Log Files
Shrinking a Database
You might also consider increasing the frequency of you log file backups.
hmscott