On a different note, I have heard it being said that it is better to overestimate than underestimate. This is apparently the best way to manage other people's expectations and maintain credibility.
And yes, it does take time to build that trust. Often times when I'm at a new company, the business people are used to short estimates and overruns. I have to train them to understand that while my estimates are longer, they are typically closer to correct. It only takes once or twice of having something done when expected to win them over.
I generate realistic estimations by attempting to overestimate and failing to do so sufficiently.
Watching coworkers try to estimate how long it'll take to deliver software really makes you wonder if pessimists actually exist, or if they're rather some kind of myth or mirage.