Search engine optimisation is pretty much part and parcel of creating websites these days. Getting the basics right don't require a large amount of work, but can make all the difference. Many of these also improve the general usability and accessibility of your site too - so you get three major benefits rolled into one.
The points below are just a few basics I've picked up along the way; roughly in order of my own priority, relating specifically to technical changes you can make on your own site, regardless of any incoming links, frequency of page updates, or link building you may do; I'm not claiming this is a comprehensive list by any means - but I'd be very interested to from others as to what they've found.
1. It's all about the page title!
If you can't spare time to do anything else, at least do this! Ensure there is a unique title for each page on your site, and make it as keyword rich, and relevant to the content on the page as you can. If you always include, say, a company name and tagline in the page title, that's great - but it's generally a good idea to ensure the portion relevant to the page appears first. Seach engines truncate the titles, so it makes sense to have the most relevant and useful information at the start - and the consistent company name and tagline at the end - for both users scrolling through pages of results, and for the engines themselves.
2. Think about the text being used to link to pages
We've all seen Google "bombing" in action - the most famous being "miserable failure" taking you straight off to the white house. Interestingly, Google has now pulled the plug on that particular quirk. However, the general rule remains the same - if a search engine picks up a link with the text "great .NET products", even if the page itself doesn't mention those particular keywords, you can still be listed in those search results.
If you find yourself creating a bunch of hyperlinks along the lines of "Find more here", "more information", "click here" - you're wasting a potential goldmine of keywords, and from an accessibility angle, each distinct URL linked to from a page should have distinct text associated with it anyway . Think twice as to whether you can include some more relevant keywords in the link text, such as "Find more developer jobs here".