The technique of building the funnel to point to a "sign up for our mailing list and we'll let you know when the product is ready!" link. You get valuable customer insight from the funnel, and if your marketing funnel is working right then you'll get interested customers in your mailing list. If it's not then you can iterate on it, and find something that works (or fail fast if you can't find anything that works).

I'm curious if you still see this working well these days, or what techniques you use to make it work well. I've historically found those types of pages quite useless when we're in a time that anyone can throw up a page to try to validate vaporware and no one wants to give out emails.

I have a feeling that advice was great 10 years ago, and not so great now.