...experiment has shown that the more detailed subjects' visualization, the more optimistic (and less accurate) they become.... A similar finding is that experienced outsiders, who know less of the details, but who have relevant memory to draw upon, are often much less optimistic and much more accurate than the actual planners and implementers.
So there is a fairly reliable way to fix the planning fallacy, if you're doing something broadly similar to a reference class of previous projects. Just ask how long similar projects have taken in the past, without considering any of the special properties of this project. Better yet, ask an experienced outsider how long similar projects have taken.
You'll get back an answer that sounds hideously long, and clearly reflects no understanding of the special reasons why this particular task will take less time. This answer is true. Deal with it. - Overcoming Bias (a blog)
Saved here for my future reference.
I've been working on my estimation abilities for work. I'm getting better. I'm getting closer to a routine 20% overflow, rather than the more standard "wrong by 2x or more", but that last 20% of optimism is proving difficult to overcome. And the only person that likes someone who routinely over-estimates is Captain Kirk, who is, you will note, an entirely fictional construct.