DO ask your local bookstore if they have it on order. If your local store is part of a large chain, such as Borders or Barnes and Noble, the odds are very good that the answer will be "yes," and that they'll be more than happy to hold one for you. If your local store is small, and does not focus specifically on science fiction/fantasy, they may have been waiting to see signs of interest before placing an order. Get interested! Interest is awesome!
DON'T berate your local bookseller if they say "no." Telling people they're overlooking something awesome doesn't make them go "gosh, I see the error of my ways." It makes them go "well, I guess it can be awesome without me." Suggest. Ask if you can special-order a copy. But don't be nasty to people just because their shelves can't hold every book ever written.
DO post reviews on your blog or on Amazon.com. Reviews are fantastic! Reviews make everything better! Please, write and post a review, even if it's just "I liked it." Honestly, even if it's just "this wasn't really my thing." As long as you're being fair and reasoned in your commentary, I'm thrilled. (I like to think you won't all race right out to post one-star reviews, but if that's what you really think, I promise that I won't be mad.)
DON'T get nasty at people who post negative reviews. You are all people. You all have a right to the ball. That includes people who don't like my work. Please don't argue with negative reviewers on my behalf. It just makes everybody sad. If you really think someone's being unfair, why don't you post your own review, to present an alternate perspective?
DO feel free to get multiple copies. No, you probably don't need eight copies of Rosemary and Rue for your permanent collection, but remember that libraries, school libraries, and shelters are always in need of books. I'm donating a few of my author's copies to a local women's shelter, because they get a lot of women there who really need the escape. There are also people who just can't afford their own copies, and would be delighted. I wouldn't have had half the library I did as a teenager if it weren't for the kindness of the people around me.
DON'T feel obligated to get multiple copies, or nag other people to do so. Seriously, we're all on budgets, and too much aggressive press can actually turn people off on a good thing. Let people make their own choices. Have faith.
DO suggest the book to bookstore employees who like urban fantasy. Nothing boosts sales like having people in the stores who really like a project. If your Cousin Danny (or Dani) works at a bookstore, say "Hey, why don't you give this a try?" It just might help.
DON'T rearrange bookstore displays. If the staff of my local bookstore is constantly being forced to deal with fixing the shelves after someone "helpfully" rearranged things to give their chosen favorites a better position, they're unlikely to feel well inclined toward that book—or author. It's not a good thing to piss off the bookstores. Let's just not.
So those are some do's and don't's. I'm sure there are lots of other things to consider; this is, at least, a start.