If you've done 6 months of SEO for a site and yet there haven't been any improvements, how would you go about diagnosing the problem?

I might approach the problem as if it were an entirely new project. Again taking a look at the keywords and phrases that I am attempting to optimize for and again taking a close inspection of top ranking competition. If the site is indexed and does show up in the irst 10 pages of a search but on in the top three, I would look into modification of major areas such as page titles, on page text, and page descriptions. If the site is not yet indexed or has been dropped from an index, there are major problems and the site may require a total re-work and re-submission.