You can configure alerts for each database as per Steve's link and set them to 80% of your database size. So that will alert you when your database reaches to 80% and also configure adhoch log backup job. Get them triggered with database size alert.
So you will be alerted when database size reached @ 80% and adhoch log backup job will execute at the same time which will database log backup and reduce the unused space.
Please revert if any doubt...