Since my first book won't be out for a few months (until July 2007) it should be obvious that I don't have long years of experience with promoting, but I have been querying other writers. It's a mixed bag out there, and no one has the keys to the kingdom.
However, the first thing you need to consider, and I mean really consider, is your audience. If you're trying to reach younger folks, realize that attention spans are very brief. Even if they see your name and book title over and over again, if they can't buy it when they see the ad, it's pointless to advertise. This is particularly true of e-books.
That's where the main "decision point" is going to be. Are you coming out first as an e-book or as a paperback, or even as a hardcover? Because those things are going to affect who--and when you need to first advertise.
For all authors, make sure you coordinate any reviews, advertisements and other promotions with your publisher's marketing department and editor. You don't want to give them the idea that you are going around them. This is a team effort. You are part of a team. You don't want to send a copy of your book someplace for review, only to discover your marketing department has already done that. It's bound to make someone angry. You don't need that.
Remember: this is a team effort. Coordinate with your team.
If you're coming out first in e-book, realize that you are catering to the "I want it now crowd. That's the whole point of an e-book. So you don't want to start advertising months, or even one month, before your book comes out. If you do, by the time the book is downloadable, you'll have lost any small advantages your ad might have garnered for you. In fact, it may be detrimental because potential readers, having tried to buy your book when they saw the ad, won't be able to, and their impression will be that there is something wrong with your book or publisher's site--like--are they going out of business? It is unlikely the reader will try again.
Also, realize that your audience is on the computer. They are not in a library or a bookstore. You don't need to advertise to the more expensive publications that cater to librarians or bookstores, like Romance Sells. There is a small exception to this: if you think you can appeal to the librarians and bookstore managers as readers, who might then chat up your book to their customers, you can consider this, but on the whole, it's a pretty expensive venture for very little gain. Save these for when your book comes out in print.
Make Advanced Reader Copies (ARC) of your book and in coordination with your marketing deparment, send them to folks who do reviews. Getting someone to review your book is one of the BEST (and free) ways to get it known and purchased. Some sites prefer to download the e-book so you, again, will need to coordinate with your marketing department on how to provide a free download to reviewers. Many of them take care of this for you. In fact, most do.
As far as places to advertise, probably your best bet for e-books is Romantic Times. They often do a review (free) and will accept ads. Another place for exposure is RWA's monthly magazine. Some other ideas that are a little unusual, but sometimes work out are:
Mention the book and where to purchase it in your High School and/or College alumni newsletter or magazine. Classmates will often purchase out of curiosity or so that they can brag that they went to school with an author.
Some folks find that MySpace or other sites on the Internet like that are helpful, but they are also collossal time-wasters, so you should consider that factor as well.
These are the hard core consumer items, so this is where you can really begin to put in a lot more effort, including book signings, etc. I've been to some book signings and you should be aware that the advantage is NOT in signing the books, the advantage is in getting it mentioned in the newspaper. You may only sign 20 books, which is not going to make or break you as an author, but if the event is mentioned in the newspaper, then you may get people to buy the book the next time they go to the store (or online).
You should schmooze librarians and booksellers--particularly the booksellers. If they don't acquire your book and put them in their store, your chances of getting sold are...slim...to none. They place their orders in advance, so they need to see information about you and your book well in advance of it's initial release. This is where publications like Romance Sells are invaluable. ARCs are important, and if you can get a few ARCs to buyers for a few chains in your area, so much the better.
The advice for hardcover is much the same as paperback, except with possibly more emphasis on librarians. There are a lot of libraries and a lot of libraries buy a lot of hardcovers. So make sure you and your marketing department get that covered as much as possible. Also, attend conventions where there are librarians and booksellers. Make friends with them. Give them ARCs. Do what you can to find out about who does the buying and how decisions are made. Encourage them to make positive decisions about your book.
Finally, I want to emphasize again that you need to work hand-in-hand with your marketing department. You aren't in this alone. They can help you. They want to help you, and they know what they are doing.