Good authors are constantly researching. Even you are just mentioning what kind of sidearm your character carries, it better be believable.  Even though Dan Brown got caught in the arms of controversy regarding the Da Vinci Code, it was his research into the Catholic Church and secret organizations that drew him so much attention.  In the same vein, new authors of Regency Romance (one of the top Romance sub-categories) are rejected if they even get a clothing detail wrong; their readers would know the difference.

(Bonus: This means taking children to the museum is a tax write-off for research purposes!)

What information can you offer your clients that they can't get elsewhere as easily?  What details do you provide to give you authenticity?

Tomorrow: Honor thy fans; they will sell for you.