"Two weeks!"

Seriously. My first estimate is always two weeks. Because I have some sort of bizarre mental block that makes me think everything sounds like it'll two weeks.

I try to work around it, try to really think about how long I think something will take, trying to identify all the potential trouble spots and bits that look too black-box-y for me to be accurately estimating. And try to recognize that if my answer is "Two weeks!", I've likely failed to do so.

Pretty much every good manager I've had has learned to recognize "Two weeks!" as an answer that requires a mild verbal pimp-slap in response.