Not a niche, How would things be different.

I understand that many people here run niche sites and networks targeted, my scenario is different. I run a modeate sized portal with true content and editorial staff of nearly 10 people. We get over 20,000 unique visitors per month.
How would recommendations change for doing SEO for such a site. The site already has a PR of 4, last year it had a pr of 5 and more visits per month. This is why I want to do SEO to get things back up.

First determine the keyword sets you want to rank for.

Then optimize on the site: interlinking structure, page/article template, etc.

Next optimize the articles, and teach your staff how to write them optimized from the start.

Finally, get backlinks to your homepage, sections/categories and individual articles that you want to rank.

In other words, the technique is no different - only the scale is.