(I had to stop writing at some point. I'll probably continually revise this. Keep in mind, this isn't meant to be a comprehensive lesson in SEO. It's meant to help demonstrate how simple SEO is and how to spot the people who have strayed from that reality.)
I actually don't like any of the websites recommended by the above users for the following reasons:
They're in it for the money. They don't do this because of their altruistic nature. They're out to make a buck. They don't care about you or your website and will do whatever they can to get you to part with your money. (This ties into the next item).
They make you think SEO is changing constantly. Guess what? It isn't. But they have to constantly churn out theories and tools and nonsense in order to make you think that SEO is this big complicated thing that requires "experts" like them and expensive books and tools and constant work and gathering of information in order to succeed. It's simply untrue. (Every time you visit their site looking for the latest hot tip or trick (barf) you're making them money).
They're wrong or behind the curve a lot. Because they're job is to constantly spew out "information" they tend to offer advice that is misguided or flat out wrong. (I don't have any examples handy but if you look around it isn't hard to find). But they don't care if they're wrong because it isn't their bottom line they're hurting and they can always claim "SEO is a changing game" and it's not their fault they were wrong.
If you check these forums, or anywhere I lurk for that matter, you'll see that the only SEO guru that I recommend (or even call guru) is Chris Beasley. Not only does he have a long history of getting it right (and before anyone else does, check out his site and the SitePoint SEO forums) but he doesn't pretend that SEO is this big complex constantly changing game. It's zero BS SEO advice. Take it or leave it. He doesn't really care what you do because he's not trying to get rich off of people's ignorance. He's glad to share his knowledge and if you take advantage of that, cool. If you don't, oh well.
As for how you can tell what's good SEO advice and what's not? Here's two litmus tests you can use. One for the source of the information, one for the information itself.
SEO Source Litmus Test
Are they trying to profit from the information? If "yes" it's less credible.
Do they have any kind of testing or other empirical evidence that backs up their claim? If "no" it's less credible.
Does the person offer logic and reason to back up their conclusion that doesn't violate any of the SEO Information Litmus Test items below? If "yes" then the information is less credible.
If the advice is in a forum or some other community, what kind of response does the user get? Lots of agreement? Or disagreement? Who is agreeing/disagreeing? If it's a newbie who is only supported by other newbies (the "me too" people) then it's less credible.
Was an experiment performed? If so was it done well? (Did they have a control group? Did they rule out other external factors? Can they repeat it)? If not they are less credible. (This is a good example of good testing).
SEO Information Litmus Test
SEO is really just making sure a website is built properly. Yes, it's that simple. And it can be broken down into four categories. They are:
Usability - People are stupid. We all know that. A good website will assume every user is stupid and needs to have their hand held as they use the website. This means everything is clearly labeled and flows in a logical manner. How does this affect SEO? If everything is clearly labeled and flows in a logical manner then search engines will also be able to follow it and understand what it is doing and where it is. Needless to say that's very important when determining what a page is about and how everything relates to each other.
(What falls under usability? cross-linking internal pages, sitemaps, proper page and heading text)
Accessibility - A well-built website can be used by anyone. This includes visually and physically impaired users. So, you need to make sure your website can be used by someone with no JavaScript, images, CSS capabilities, mouse, etc. How does this affect SEO? Search engine bots are not much different then a blind user. In fact if you use a screen reader you can get a pretty good idea of what Googlebot is experiencing. So if your site is accessible to impaired humans, it will be available to search engines.
(What falls under accessibility? alt attributes, not relying on JavaScript for content)
Semantic markup - Any experienced web developer will tell you that separating your JavaScript and CSS and using the proper tags for markup makes the site much easier to maintain and offers external tools (directory crawlers, etc) the information they want quite easily. Plus if you use proper markup your site is "futureproof" as you can be sure standards compliant user-agents will render your site properly. How does this affect SEO? HTML markup is a big indicator to search engines as to the meaning and value of content. Using the proper markup allows any user-agent to comprehend data format and structure. It tells them what's important and what kind of information it is (knowledge, address, table of contents, etc).
(What falls under semantic markup? tags like <address> and <abbr>, using heading tags (<h1>), microformats)
Content quality - People aren't surfing the Internet looking for garbage. They're looking for information that solves a need they have (to know something, to be amused, etc). The better the content the more likely they are to come back and, even better, tell their friends. How does this affect SEO? This is how you get quality links to your website!! Search engines have difficulty telling quality from crap. So they rely on other webmasters to do it for them. That's why links from external sites carry so much weight. (We'll save the quality issue for another day).
(What falls under content quality? original content, links from authoritative external sites)
If a piece of advice does not fall into one of these categories then it's probably just plain old wrong.
(Seeking links from other website is marketing. It can be debated if it should be listed here. I chose not to as quality content can do that for you).
Other helpful information
SE algorithms consist of two basic parts: page ranking and spam control. 99% of the people who talk about SEO do not know this and confuse the two which causes lots of problems. (FYI, Matts Cutts is the head of the Google Spam Control team). So when people say "SEO is constantly changing" they're wrong. How pages are ranked has barely changed in years. What is constantly changing is the search engines' ability to fight and remove spam. If spam fighting techniques is affecting your site then it isn't affecting your SEO, it's affecting the fact that you have a crappy website in some way or market your website improperly and need to address those issues. But the basic factors for ranking pages was figured out a long time ago and hasn't needed to change. It's how the spam is handled that needs to change (and is).
If you have to go back and "SEO your website" then you built it wrong to begin with. Those four basic principles aren't SEO factors, they're how a website is supposed to be built. All users want is a website that they can easily use and has good content. It's no coincidence that search engines want the exact same thing.
Ask yourself this question: Am I doing this because it will make my website better or am I doing this to manipulate my search rankings? If you answer the former you'll be alright. If you answer the latter then the odds are it's going to bite you in the butt somewhere down the road.
