I don't have just one tip, I have several:
1. make sure your book sparkles. Ahem, I inadvertently had a bit of trouble with this recently. It can lead to a lot of trouble. So make sure you use a reputable editor and perhaps reread your edited work. Just a suggestion.
2. Stay away from reviews. All of them. Don't read them and be very, very careful writing them. It didn't take me very long to realize how quick reviews can lead to trouble. Reading them can kill your passion for writing and writing them can put you in a weird position because people may then either:
a. start e-mailing you to solicit reviews--hey, you wrote so-and-so one, why not me, too?
b. if you give anything less than a glowing 5, some, not all, but some, people think you're a jealous author trying to hurt someone else's book sales--this ends badly. On the flip side...
c. if you ONLY give good reviews, people either discredit you or gripe because you only give good reviews--there really is no winning in this situation
3. Another thing on reviews NEVER, EVER respond to one. If you do, you lose. Don't thank people for them, don't ask them to elaborate, don't defend yourself, don't ask their advice, just DON'T go there. (And don't ask a friend to do it for you, either.) If you comment by a review, you lose all credibility. Just let it be. If another reader jumps to your defense, great. But don't you dare do it.
Other than that, find your niche and try out every different marketing tool available to you to reach them. Facebook, twitter, blogging, guest blogging, ads, whatever it takes.
Best of luck.