A while ago I posted something on this subject, but - I wanted to let everyone know that Mike Lagase has just posted a very detailed troubleshooting guide for those problems on his blog. This is the most comprehensive collection of information on how to troubleshot those issues that I have seen, internally or externally.
To see it, go here:
Troubleshooting Store Log/Database growth issues http://blogs.technet.com/mikelag/archive/2009/07/12/troubleshooting-store-log-database-growth-issues.aspx
- Nino Bilic