You can use your website to help promote it. Have links to your books on your website, and then build in suitable keywords into your website. Put a link to your website in your sig, and post on a wide variety of boards.
Also, the biggest thing is word of mouth. Get people to read it and to tell other people. Get them to write reviews.
Get onto your local papers and radio station and get them to do a story on you.
If your novel is on Amazon, you could try dropping the price so it will appeal to impulse buyers, so it will rise higher in the rankings, and more people will buy it.