There are two aspects to improving your WordPress SEO. The first is configuring WordPress, using inbuilt settings and WordPress SEO plugins, to gain maximum SEO benefit from the platform. The second is to ensure that your content is properly optimised so that the search engines understand that it is highly relevant to the chosen keywords, this of course is the same for any site.
Configuration
SEO of a standard WordPress installation can be improved by making a few tweaks. In most cases the easiest way to do this is by installing WordPress SEO plugins which make it easy to make changes without editing any code. However the first key changes are easy to make in WordPress itself:
Go to Settings - Permalinks and change the "Common settings" to Custom Structure /%postname%. This will put the post/page title in the URL instead of the default numbers, so the URL will include the keyword.
Go to Settings - General and make sure the site address is set to include or exclude www as you wish (see Canonical URLs below). It is also worthwhile setting this in Google Webmaster Tools. You want to avoid using both as this can be seen as duplicate content and you could get backlinks to both thereby diluting the benefit of your link building.
Then install the "All in One SEO Pack" WordPress SEO Plugin. This is the best known WordPress SEO Plugin and for good reason. It helps your WordPress SEO in many ways by giving you amongst others:
The ability to add a meta description and keywords for the site and each page/post - it is best to write the description manually rather than allow it to be auto generated by the plugin or indeed, by the search engines.
Control of the format of the various titles. I prefer to just have the page title showing on the search engines. You can add keywords but it is best to leave this alone.
Option to prevent indexing of Categories and other duplicate content. The structure of WordPress means that there is always duplicate content as each post is duplicated by category, tags, date and author.
Control of URLs. Make sure that Canonical URLs is ticked as this avoids having duplicate URLs for the same page.
You can also add WordPress SEO plugins which for example:
Shorten your ULRs by removing unnecessary words
Cache the pages/posts to speed up page loading
No-Follow external links
Add Internal Links automatically
Make commenting more user friendly
Content Optimisation
Whilst the above all help to maximise WordPress SEO it is important to emphasise that the most important task is to ensure that the content is properly search engine optimised by, for example:
Using the keyword in the title, header tags, first and last sentence and at a reasonable density throughout the text.
Ensuring each page/post includes an image and using the keyword as the ALT text.
Having links to other pages on the site.
Having external (no-follow) links to authority sites.
One way to check this is to use a WordPress SEO plugin that analyses your content and tells you want needs to be changed.