Rand Fishkin’s SEOMOZ has been doing some of the best work collecting data from prominent SEO folks and groups of experts and then analyzing that data. Back in April I missed this report about SEO ranking factors but it’s a great read, especially for those who have little idea about how to optimize a website and web pages for better placement in search engines. Note that experts do not agree. Also, my fairly extensive experiences have convinced me that Google changes the ranking rules regularly simply to make it impossible to reverse engineer them. But it’s still important to follow these basic recommendations which include what I’d argue are now the “prime directives” for optimizing websites:
Create pages that are of high and unique content quality.
Use URLs and Titles that are highly relevant to the queries you wish to rank for.
In bound links are still very important – seek external links and create internal incoming links using your desired keywords as anchor text.
Tend to exaggerate the keywords you are targeting. ie the best writing will NOT result in the best optimization due to defects in the way machines process word information.
Joe Duck 
There’s no need to exaggerate your keywords, Joe :-). Google + co are pretty good at picking out the important ones, even if they’re only mentioned once. However, it does make sense to choose your words in a way that will help to get your content found again.
For example, you might describe your first ski trip in Switzerland like this: “This old dog finally learned some new tricks. The powder was great but I turned into an icicle going down.” The writing might be creative (sorry, I’m not particularly good at it), but there’s no way that a search engine would associate “ski trip” with it (at least not directly from the content itself).
So choose your words carefully, but there’s no need to exaggerate, there’s no need to work on “keyword frequency”.
John I don’t think I agree with you on this one. I’m basing some of this on my read of the ultimate expert in SEO, Matt Cutts at Google:
It seems to me he has often suggested that it helps to really hammer home the point to the search engine that your page is about your keyword:
http://www.mattcutts.com/blog/seo-advice-writing-useful-articles-that-readers-will-love/
Note in his SEO Optimizing example about Linux printing that he uses terms often – to my way of thinking he is exaggerating them but I guess that could be debated.
He does note that you should avoid “keyword stuffing”:
http://www.mattcutts.com/blog/avoid-keyword-stuffing/
Of course there is a balance here, but I’d recommend people err on the side of helping the bot understand your page by emphasizing the keywords that accurately describe the page.