Link Building
Link building is one of the, if not THE, most important parts of SEO. The more high-quality, trustworthy sites link back to your site then higher it will rank. There are many way to get backlinks.
One of the most obviously things to think of is that people should WANT to link to your site. If it contains useful information, great products, useful tools etc then people will automatically want to link to it. This is often called 'link bait'
You may have seen sites where they have a little Flash/Shockwave game on them or some little amusing video. This are great for linkbait. Someone mails a link to the video to a friend, that friend puts the link on his blog saying how great or funny it his... his friends put in on their blog or put a link to it on their Facebook page... etc. The effect snowballs. This is often called Viral Marketing.
So what else can i do?
There are many free directories and paid directories you can easily get your site in. Unfortunately many(most?) of these provide little more than a fairly low quality/weight backlink from some deeply hidden page. If a directory has a high PageRank and good traffic ranking then it may well be worth paying for a 'feature listing' if something like that is available to get your link to actually be visiting and not buried at page 139 etc.
You can also trying creating your own blog. Write regular articles which link back to your site. You can even put these articles on your site.
Social bookmark sites are also a great way of
Link Building. You can't really just bookmark your homepage but you can certainly bookmark your content and article pages. Obviously many social bookmark sites won't let you bookmark particularly commercial content pages so try and create some purely informational pages.