To gain an appreciation for quality score, I recommend that that you begin with this YouTube video titled “Introduction to the Google Ad Auction”. It’s very well done.

Depending on your level of interest, I have compiled a list of additional resources, which I think you will find informative. They include targeted sections from the AdWords Help Center, as well as some outside resources I’ve compiled.

What is 'Quality Score' and how is it calculated?

What Determines Quality Score?

Understanding & Improving Google AdWords Quality Score

10 Ways To Increase AdWords Quality Score

Websites that merit low quality scores

As it relates specifically to customized landing pages, here are a few best practices:

• Have the keyword in the landing page URL.
• Keyword rich meta tags for:
  • Title tag
  • Description tag
  • Keyword tag
  • Image alt tags
• Strive for a keyword density of 2-3%, but don’t sacrifice salesmanship in the process
• Avoid
  • Pop-ups
  • Opt-in forms
  • Auto-play audio
• Make sure the landing page loads quickly (load time). I don’t recommend using a blog as a landing page because it takes too long to load and may have a lot of external links that lead the visitor away.
• That said, I do recommend having a few "low key" links at the bottom of the page out to some authoritative sites on the subject (for example Wikis, .edu sites, etc. that you’d find on top of the free search results). These low key links don’t distract many of your visitors, but boost your quality score.
• Generate incoming links to your site with the keyword phrase in the anchor text. This isn’t absolutely required, but will help in competitive markets. Some ideas for getting a few incoming links quickly are submitting articles with your link in the resource box, post comments on blogs in your market, and link from your other sites.
• If you have several landing pages which have a lot of duplicate content, don’t put them on your navigational structure, place them in a sub-directory. If you are concerned about the possibility of duplicate content from an SEO perspective, use an appropriate “no index” script.
• Make sure there is no code on the page that prevents the Google bot (AdsBot-Google) from spidering the page
• Prominent links to
  • Privacy policy
  • Terms of use
  • Any disclaimer
  • Contact us and include:
    • Company business name
    • Street address
    • Phone number
    • A real person’s name is also helpful
  • An escape to the main body of the website
• Make sure Google is interpreting the site as the intended category, (the correct site genre). Check this with the Google external keyword tool and type in your website, then note the categories on the left side.
• Get a couple quality inbound links to the site that are from related types of categories and make sure the hyper-text is the keyword you are optimizing for.
• Have a couple of outbound links to authority sites with a page rank of at least 4. Websites with a .org tend to be good because they tend to be authority sites, but not competitive.
• Create a blog in a separate directory on your website with a link to the blog on every page. Continually update the blog with fresh, relevant content. However, do not use the blog as a landing page because there may be too many outbound links.

Now, here is a real pearl of wisdom that trumps almost everything I've said. I have found that the overriding factor in a keyword quality score is "how that keyword has performed throughout the Google system". This is a direct quote from Google, which they understate.

Therefore, if you have a relatively short, broad keyword such as "divorce attorney", it is going to be very difficult to have a decent quality score. By decent I mean 7 or higher. In fact, it will be nearly impossible. The implementation of the quality score metric is Google's way of not only insuring relevance for the user, but it's a profit dial for Google because they can control how much an advertiser must pay to compete by simply manipulating the quality score algorithm.