What usually eats up space
Large files sent repeatedly, multiple versions of the same document kept out of caution, or documents from long-finished projects that were never deleted.
How to see the problem coming
The storage usage bar clearly shows how much of your plan's limit you've used, before you end up unable to upload a new file at a critical moment.
What to do as you approach the limit
Check documents from long-finished projects, they can often be archived or deleted without real loss. If the need is constant, a plan upgrade is less costly than the time spent manually managing space.
Bottom line
Managing storage isn't urgent if you check it periodically, it only becomes urgent when you ignore it down to the last few MB.