If you are looking to do SEO on your site or are being asked to do SEO things for your company – going at it in the right way will really help you out. There is a reason that there is a demand for SEO firms – it's not something you can pick up and learn in a day or even a year
Start With The Basics
If it's a new site (i.e. not yet open) you need to consider the fundamental SEO concepts while the site is being developed. If the site already exists, you have some more work to do, but it can be done.
Be careful if you are changing URLs or the location of the various pages within the site – do your best to 301-Redirect anything you move to the corresponding new location. And, if possible, do it so that each redirect goes to the specific page that it used to have – not just avoiding a 404 by redirecting everything to a home page.
Page Titles: Arguably the one most important thing to SEO is the page title. It needs to say what the page is about, inspire searchers to want to click that link, and it should have the primary keywords in it to help it get to the top of the search. (Take a look at my "Own Your Own Brand" series of articles for some insight into helping with the decision of whether or not to include your textual brand symbol in your page title too… this can do a lot to help increase your own exposure even for searches where you appear, but the person never clicks).
URL Structure: I hate hate hate URL rewrites. Sadly, though, with dynamic content generators like blog software, content management systems, and so on, they are quite often necessary because the developers didn't really take the time to think it through before they started building the application. My URL Rewrites Article will give you a bit of background on this topic.
Once you have the basic ideas down, do a little research into the specific software you are using and find out (usually support forums are great for this) what other people are using/doing in order to get the most effective results. (NOTE: Just because something has an "SEO" plugin available, it doesn't necesarily mean that that is actually going to do you much good. Most that I've seen do not hurt, but many don't really do anything useful other than maybe get some keywords in the URL or help a bit with page titles… these things are important, but shop around and look – or talk to your own developer and see what kind of hacks they can do to make it really do some useful things).
Navigation and Link Structure: Understand that SEO is as much about what is on every page as it is about how other pages that link to it and that it links to relate to each other.
Read, Read, Read – And Then Some
In keeping with the statements above, you should start out with reading things about the basics and getting a general overview of things: Ammon Johns' "A Quick Kick-Start Guide to Search Engine Optimization" is a classic post on Cre8asite Forums.
Once you have the basics down and once you have your site set up in a way that it will handle the basics for you, then (and only then) should you start looking into the more advanced techniques.
Build the house on top of a solid and established foundation. Don't start with the little tricks and try to work down – the house will get in the way.
Avoid The Fancy Stuff For Now
Plan For The Future
Web sites are evolving entities.
If the site is something that is just going to be put up and never touched, it'll end up having a limited lifespan. You can optimize it and it'll do well – until someone else comes along and does something a bit better and a bit more inclusive and finally nudges you down and out.
Virtually every web site should have some sort of area that is updated with new content, new ideas, and something that says to users and search engines: "Hey – we're here and we're still doing great things! Come and see!"
As you plan for the things that will be added to the site, create a good SEO checklist and template that will do most of the mechanical work.
If you are doing something that uses tag clouds or something similar – make sure that every new article and post gets the proper tags in there. Make sure that as older articles fall off the front page they are automatically archived in a way that search engines (and people) can still find them.
And finally – make sure that everyone on the team understands how their job is affected by SEO and what they need to do to ensure that it happens. Help them become efficient and then all you need to do is keep up with the techniques and apply them to the template so the new things you are trying all fall into place.