ForumsAdvertising, Sales and Marketing

I completely agree with Karen
Selecting your keywords is one of the toughest and most important aspects of the SEO
Doing it correctly is like walking on a thin cliff edge, one step in either direction could bring a fall (of visitors)

Even though, I do feel to stress the importance of links... having better PR means your keywords will get better results in search engines. after all, what good would a keyword do if, when searching for it, you're on the 5th page.
Without counting on linkfarms (which will eventually downgrade your ranking), I suggest having resources to create viable and good links for your costumers.
Of course for every market you'll need different resources, but you could always find good general places that will apply for everyone
One of these places is individual pages.. you could offer to enter these people into social networking sites such as LinkedIn and Naymz. services like LookupPage could also come in handy.

Link building is, I think, a form of art. though that can be said about SEOing in general..

Good luck with your business
Jmes