When projects run for many years, they require special regular maintenance to stay fresh. This kind of maintenance is usually not necessary for small or quick projects, but required to keep long-running projects from getting stale.
You should be able to fit this into a regular development block.
As time goes by, libraries outdate. Check your software components and decide if any of it needs an update. Your main components (e.g. Ruby, Rails, Angular) should always be reasonably up to date.
It is usually inevitable to defer some tasks that "would be nice to have" but have no room in the project schedule. Check your list (e.g. the Pivotal Tracker icebox) and decide on what to do: some technical debt might require imminent action, some wishes might already be forgotten and can confidently be deleted.
Some table may grow more than you expected. If they do, dump size will increase (making backups and other dump operations more expensive) and your application might suffer from reduced performance.
Just like database tables, storage usage may increase faster than expected. Check file storage locations of your application and do a quick sanity check by examining disk usage and folder structure. External storage providers like Akamai Netstorage are paid by the gigabyte, so you should aim to keep your footprint small.