Ideally, your title tag should contain between 10 and 70 characters (spaces included).
Make sure your title is explicit and contains your most important keywords.
Be sure that each page has a unique title.

Meta descriptions contains between 70 and 160 characters (spaces included).
It allow you to influence how your web pages are described and displayed in search results.
Ensure that all of your web pages have a unique meta description that is explicit and contains your most important keywords (these appear in bold when they match part or all of the user's search query).
A good meta description acts as an organic advertisement, so use enticing messaging with a clear call to action to maximize click-through rate.

Gzip is a method of compressing files (making them smaller) for faster network transfers.
It allows to reduce the size of web pages and any other typical web files to about 30% or less of its original size before it transfer.

Redirecting requests from a non-preferred domain is important because search engines consider URLs with and without "www" as two different websites.

A sitemap lists URLs that are available for crawling and can include additional information like your site's latest updates, frequency of changes and importance of the URLs. This allows search engines to crawl the site more intelligently.
We recommend that you generate an XML sitemap for your website and submit it to both Google Search Console and Bing Webmaster Tools. It is also good practice to specify your sitemap's location in your robots.txt file.

A robots.txt file allows you to restrict the access of search engine robots that crawl the web and it can prevent these robots from accessing specific directories and pages. It also specifies where the XML sitemap file is located.
You can check for errors in your robots.txt file using Google Search Console (formerly Webmaster Tools) by selecting 'Robots.txt Tester' under 'Crawl'. This also allows you to test individual pages to make sure that Googlebot has the appropriate access.

Your site's URLs contain unnecessary elements that make them look complicated.
A URL must be easy to read and remember for users. Search engines need URLs to be clean and include your page's most important keywords.
Clean URLs are also useful when shared on social media as they explain the page's content.

Embedded Objects such as Flash. It should only be used for specific enhancements.
Although Flash content often looks nicer, it cannot be properly indexed by search engines.
Avoid full Flash websites to maximize SEO.

Frames can cause problems on your web page because search engines will not crawl or index the content within them.
Avoid frames whenever possible and use a NoFrames tag if you must use them.