It seems that everyone who has an online business has to come in contact at some point with Seach Engine Optimization because search engines are a resource you can't afford to ignore. Because they'll give in return of your optimized sites, quality targeted traffic for free.
Comparing this to PPC, you can consider it a long term investement which is often desirable if you plan for a long term business.
It's not that easy to do SEO. After all, you're trying to trick the engines to some extent and they will have something against that. You can get lucky and get your term/keyword/phrase to rank excellent on Google but you might not have any idea what exactly did you do right to make the same thing again. Do a mistake and you get penalized, banned and again, you might not have any idea what exactly did you do to avoid doing that again. Some would say it's an art. I wouldn't go till there.
How would I start learning about SEO again now after 3 years of experience? Learn about on-site optimization and apply it. Make 2-3 test sites. A niche, a blog, a directory for example.
Learn about off-site optimization: search for Matt Cutts videos and watch them. Read his blog. Go through SEO forums and read them (don't get lost though, there's a lot a garbage on the forums). Get the SEO Book if you can spend some money. While reading make a list of free SEO techniques and a list of paid SEO techniques. Start with the free techniques and see how good each of them is by watching Google's feedback (or Yahoo or Live).
After the sites pick up some momentum, think of how you can monetize them and invest in paid promotions. Keep in mind, paid is not always better. It's usually just faster. Invest in some niche promotion: you have a travel business? Submit it to travel directories or categories - Travel Business.
Always research, always stay updated. Try as many things as time allows you. Outsource!