What makes a search engine friendly site?

Hugh is currently working on a new site, so he obviously wants to get it right from the beginning.
Like so many other “net savvy” entrepeneurs Hugh knows that the search engines are crucial to his venture’s marketing strategy. If the spiders don’t like his site it may not be as succesful as he would like.

So what is the secret?
How can you find the grail?
Of course a glance at the metaphors give it all away. It is the “grail”, the ultimate challenge. Some rise to meet it, others die trying.
A few months ago I conducted a couple of experiments with SEO when “playing” with a blog. I played around with the position of titles, the juxtapositioning of keywords and phrases, mod_rewrite etc., etc. Links from sites that already had regular crawls were placed – in short I used most of the “tried and trusted” techniques.
Was I successful?
Maybe to some degree, maybe not at all.
SEO is part science, part art, part luck. No matter what you do intentionally you still seem to find that your accidents are bringing in more organic traffic than you would have expected.
For example this blog. Although I may have put some effort into basic SEO I haven’t really bothered doing a lot of actual work. I may post on some topics more than on others and I am all too aware that I will never be able to make any cash from google ads, affiliate links or anything else in this space. However, having said that, I am always amazed at what the engines pick up on.
For some odd, quirky reason I keep on getting top results for the most outlandish queries:

• apache2 configuration – what do I know about that???
• skype ubuntu (or derivatives) – see above question
• sugarcrm outlook

The only response that can possibly evoke is “huh?”