Organic Backlinks especially quality backlinks..... SOcial bookmarkings, forum postings, Free Promotional Ads, etc...
Best method: Have clients, take their money, and ask beginner questions about what you're selling them. That always works.
Twalters is right but I have some comment on what he says. Do first the on-page optimization before you proceed to off-page optimization. It's not worthy if you directly go to off-page without optimizing first the on-page.