Does anyone have a good approach for measuring the success of your efforts to chip away at technical debt? What KPIs are you using?

I like to use the platform lifecycle KPI (Application + O/S version - support life (years) / depreciated value of CapEx). To hep understand what level of risk are we able to endure in addition to the talent (internal or external) needed to maintain or evolve the platform.

We measured applications and operating systems versions.  We had percentages on all of them for N and N-1 up-to-date.  If you were N-2 (two versions away from latest release) you were yellow.  If your version was within 1 year of no longer having patches you were "Red" on the dashboard.  These metrics were reported to the CIO's of each business every quarter and to the CEO once a year as part of the IT Health assessement  (two slides out of about 40 covering all of IT)

These are very subjective to the domain of tech debt but my approach is:
- draft a "twitterverse" summary of the debt and prioritize with other debts and approved KPI's goals
- outline the risks, hurdles, costs, and associated impact on the business if you continue with the status quo
- research very creative solutions with startups, free tools/libraries, etc
- challenge the teams to come out with cost-effective solutions via hackathons
- finally, propose the best course of action up and down the food chain

Easy right...?