There are then per-application technical debt pages for specific details on our "portfolio" of debt. (We're also a fair bit larger than most startup dev teams, so having this level of formalism is more appropriate in our case than say just a whiteboard somewhere...)
We don't do anything magical; there is no secret sauce, noe no spreadsheets with formulae or anything fancy; it's literally just a series of wiki articles and Jira tickets with issue type "Debt" so we can easily find the documented debt and make plans when/whether to retire it.
So the concrete formal mechanism is "use your bug tracking system and flag them somehow so you can filter/search for them."
I will add some additional links that our top-level debt page references as useful:
http://www.youtube.com/watch?v=pqeJFYwnkjE (Video: Ward Cunningham on technical debt)