[Too]

A good way to enforce this is to always estimate things in steps of 2^x. Eg you are not allowed to estimate something as taking 20 days, it's either 16 days or 32 days. That way it's obvious that the larger the estimate becomes the more inaccurate it is and you are forced to automatically double to 32 if your "inner" estimate is 17 days.

Scrum has a similar technique only allowing Fibonacci numbers, I think the reasoning behind it is similar.

Obviously the receiver of the estimate should know that you are using this method, otherwise they might start questioning you thinking this estimate should only be 17 for example.