Start saving money. It's true. Publicists are expensive. The rates obviously vary with company and campaign. It may be a decent guideline to prepare to spend about $5,000 a month. It may not be that expensive or you may get a package deal, but prepare for the worst. On a good note, it may only be for a few months at most, hence the premium.
Is your story marketable? It helps if your topic relates to the newest trends. A non-fiction book is easy because it's real and people can more often relate to it - you simply write what people want to read. For a fiction story, you may need to cater to the newest buzzwords and topics that are popping up in the news. If your book relates to a popular issue, the market will receive it more favorably.
Self promote early. Get yourself known. Build an internet presence. You want to start to maintain a blog in order to build a following. Join networking sites such as Facebook and Twitter. The longer you are established in these networks, the better. It will help out big down the road to already have fans. If you're new to the internet networking sites, do not be afraid to jump in and try things out. It's the way things are done now and you have to accept it. Join these networks. (note: Since you're building an image, your online reputation is huge. Keep an eye on your content and stay out of trouble.)
Also it's a good idea to get footage of yourself. Yes, footage. A reel of you talking and interacting with people in a positive way will become useful soon.
When to call the publicist. This is a step you should take about 3-4 weeks before your galleys come out. This way, you can build a relationship with a proposal plan for the future publicity campaign occurring with your book 4-6 months later. If you've been following these steps, your proposal should sound quite appetizing to the publicist. Tell them about your blog followers, your facebook networking, and any popular topics your book can relate to. You also want to suggest possible ways you plan to promote on your own. It's not up to the publicist to do all the work. You can start with more blogging and planning to visit local book stores for signings. You do your part on the smaller end and leave it to them to put you on Oprah, especially if you have a good reel of footage. Seriously.
Stay persistent. The publicist will handle the big stuff but if you keep up on blogging and online appearances, you will increase your influence to readers and ultimately make more sales. Keep at it. Throw the link to your website on every blog and network you visit. Do book signings when on vacation. Tell your friends to tell their friends to blog for you. Get out there where readers can see you and they will end up buying your book!
Stay up to date. Follow the news and the blogs and check out the different books to deal with publicists. Circumstances are always changing, especially in today's economic climate. Keep up with the world. With all that done, you have a much greater chance in selling lots of copies of your wonderful book. Good luck!