Canonical URLs
Use a consistent URL scheme across the site – don’t mix http://domain.com with http://www.domain.com/ and never use a link to index.php or index.php?page=home. In the latter three cases, a link back to the web root is preferential eg. http://www.domain.com.

Search friendly URLs and absolute URLs
Using URL rewriting, create URLs that make sense to the human eye, eg. http://www.domain.com/products/sofas/big-brown-sofa.html. NOTE: NEVER use underscores in URLs or for file naming, always use hyphens. Use absolute links throughout the site eg. include the http://www.domain.com in every A HREF. When linking to a page, as opposed to an index, always suffix “.html” onto the URL. All URLs should end with “/” or “.html”.

Proper use of H1 tags
The H1 should be the main heading of the page, semantically proceeded by H2, H3 etc. Do not use H1 for the logo, use an alternative tag. Your H1 should also be reflected in the META Title of the page.

Logo naming
Don’t name your logo image Logo.jpg. Instead name it something relevant to the site – for example for a jewelry site, name it jewellery.jpg

NoFollow links
The rel=”nofollow” attribute prevents search engines from leaking page rank to useless pages, for example, for terms and conditions, privacy policy and in most cases contact pages (unless they include a physical address for example).

XML Sitemap
There is a Google sitemap generator available that will create an XML sitemap and update it on the fly, informing Google of any new pages. All sites should contain an XML sitemap as well as a XHTML one. Show both sitemaps in the footer like this:

Contact | Privacy | Terms | Sitemap (XML Version)

Images with ALT attributes
All images should have ALT attributes, primarily to describe the image for screenreaders, but they can have an SEO benefit by including site keywords.

301 redirects
Especially important when redeveloping an old site – ensure old URLs are 301 back to the homepage. A 301 redirect tells search engines “This content has moved permanently”.

Don’t use frames
We’re sure you don’t use them.. do you? Well don’t because amongst other things the content within a frame isn’t accessible to search engines. Same applies to iframes.

Check for W3C compliance
This is part of best practice and has a positive benefit for SEO.
Don’t call links page “links”
This has negative connotations to link farms, reciprocal linking etc. Instead, use a term like “Resources”, “Related Sites” etc. You should preferably come up with a term which is unique to the site, whilst still describing the page accurately.

Keep CSS and JS external
The less clutter there is in the page, the less markup search engines have to sift through. Also, avoid inline styles and inline javascript for the same reason.