Author Steve Weber gives lots of tips for online marketing in PLUG YOUR BOOK. Some that I found particularly helpful include:
1. Find readers in your target audience and give them your book. Ask them to post an honest critique on Amazon.
2. Correspond with readers who enjoyed your book, and ask if they could spare the time to write a review on Amazon.
3. Target specialized magazines and trade publications.
4. Become a "librarything author."
5. Go on a blog tour.
6. Add your book title to your Amazon pen name so that it's displayed when you post reviews.
7. Offer book excerpts or sample chapters PDF download from your website.
8. Use these pointers for writing an effective press release.
9. Auction a single copy of your book on eBay in one or more subject categories.
10. Pursue affiliate sales on websites that specialize in material related to your book.