SEO Checklist
On-Site SEO
Validate XHTML code
Validate all internal links
Keyword relevenat Title Tag
Human readable and keyword rich meta description
Meta keywords
Alternate media / Mobile version if applicable
Sound navigational structure
Popularity filtering - Link to most popular internal content on the home page
Alt tag on images
Title on hyperlinks
Keywords in page / file names
Keywords in domain
Keywords in links both internally and externally
robots.txt
XML sitemap - add to robots.txt and submit to Google webmaster, bing webmaster and yahoo site explorer
CONTENT IS KING!
RSS feeds
Add social bookmarking buttons to content pages
Publish fresh content often
Do not remove old content unless necessary, even if no longer linked to on the live site
Place your keywords in many tags - H1, H2, H3, LI, P, B
Don't build your entire site in flash unless you don't want traffic!
Follow Webmaster Guidelines
Don't link to junk
Publish content that is worth linking to
Avoid duplicating pages
Host all images and media files under your URL
Off-Site SEO
Tweet when publishing content
Blog when publishing content
Use Google alerts to monitor your keywords and contribute where you can
Use Google Analytics to monitor your traffic, conversions and keywords
Use Google WebMaster Tools
Use Bing webmaster tools
Use Yahoo site explorer
Use Google AdSense
Add links to social bookmarking tools - google notes, del.icio.us, digg etc
Submit site to DMOZ
Submit site to Yahoo Business
Manual directory submissions
Link Building
Update site info in Alexa
Additional SEO Activities
Don't try anything daft or dodgy
Research and keep up with latest SEO technologies and trends