My advice to them, and my philosophy on this topic, is that technical debt is just that- debt. It's not a dirty four-letter word to be avoided, it's something to be managed the way you manage actual financial debt. There are some projects where the software you build will be self-contained (no dependencies), will require minimal changes in the future, or it may have a short shelf life (e.g. it's a stopgap before you cut over to a new system). In these scenarios, accruing technical debt isn't the worst thing in the world if it allows you to reap business value faster.
When you're building software that's going to live long-term, or that drives core parts of your business (and therefore will need to change frequently as your business changes), technical debt needs to be tackled early and often. It's ok to accrue technical debt in the short term ONLY if you make that decision consciously knowing that you'll need to address it later (e.g. I had a client that needed to release their software for a trade show, so we accrued tech debt and created tasks to track every piece of debt we'd want to tackle later, and then prioritized that work immediately after the trade show).
That said, it's also important to be clear about what constitutes tech debt vs what is over-optimization. Outside of libraries and frameworks, I don't really need the software to be "perfect". I need it to be clear enough, tested thoroughly enough, and easy enough to change to maintain a high velocity. If a part of the code has become spaghetti-like, or if we see code duplicated more than 3 times, or if we have disparate code that's too closely coupled to easily make changes, that's technical debt worth tackling.
The vast majority of organizations I've worked with typically think much shorter term, which is why they find themselves repeatedly accruing technical debt to the point where changes to the system occur at a snail's pace. It takes a lot of discipline, and coordination between IT and business, to practice what I'm describing, but that's the ideal that I strive for and advise my clients to strive for.