The goal of these tips is to get the pages initially indexed and then more importantly, to keep those pages in the index. As mentioned earlier, just because a page is crawled doesnt mean it will necessarily get indexed, so make sure that you focus on:
1.) Getting links to the homepage from high authority, high PR sites
The same points made in the final tip for crawlability applies to indexability as well. The more links you get from relevant, high authority sites pointing to the homepage will help improve your sites authority and link funnel power. As a result the deeper levels will have a better chance of getting indexed.
Goal: To build up the authority of the homepage so that Google feels the need to index all of the deeper levels because of the homepages power. Without adequate homepage power the deep levels of a big site will not be able to stay in the index consistently.
2.) Link to specific deep level pages of the site (specific businesses/product pages instead of major hubs)
The strategy here is to go after the lowest levels of the site architecture with links. By getting links to the specific product pages or business listings (as opposed to the major hubs like tip #6 for crawlability) these pages have a much higher chance of being indexed and staying there. Now in the case of larger sites it wont be possible to hit all of the lowest levels, so its still important to hit all the major hubs, but for indexing purposes sometimes thats not enough and hence why you need to build links to the deepest levels of the site.
Goal: The obvious goal is to get the lowest levels and pages of the site indexed and have them stay in the index. Its another way of distributing link power throughout the site and especially to the deeper pages of the site because the homepage and even major hubs link power is severely diminished by the time it gets all the way down to these deep level pages.
3.) Make sure these deep level pages have adequate content
As a general rule of thumb, a few paragraphs of unique content are definitely a good starting point for these deep level pages. If possible, try and have fresh content to update these pages as much as possible in order to improve crawlability, which will in turn improve the number of indexed pages if the pages have good content. Remember, just because a page is crawled doesnt mean it will get indexed so the business listing profile or product page has to enough content to justify keeping it in the index. Oftentimes just a business name and phone number will not suffice although at one point it did.
Know its tough in many cases when you have a few hundred thousand or even millions of pages to potentially update and add content to, but try and think out of the box to combat this problem. Maybe have a reviews or comments section where users can add content themselves or something that will entice user-generated content. The content on these deep level pages is one of the biggest reasons for the page to either be indexed or not once it has been crawled adequately, so make sure that there is enough content to justify staying in the index.
Goal: To not only get indexed initially but to stay in the index is clearly the most important factor. By having adequate content you definitely increase the chances of the deeper levels being indexed. Try taking it to the next level by having fresh and updated content, and with all the other tips mentioned in the post, this will keep your deep level pages indexed for now and the foreseeable future.
Conclusion
Its absolutely essential to have good site crawlability and indexability if you want to receive optimal organic traffic from the search engines. The tips provided in this post will easily help you increase your sites crawlability, but you need to be proactive and take the next steps in order to ensure that these pages actually stay in the index.
Nowadays, major sites and business directories are starting to lose some of their deepest level pages, and the only way to combat this growing trend is to boost the authority of these sites and the way they are being deep crawled by the bots through all of the tips outlined in both of the sections. Its no longer about just boosting up the homepage PR and hoping for the best with crawlability and indexability by having a clean navigation and site structure. Those days are long gone. For deep crawling success in todays ever changing search engine algorithms, you need to vary the linking strategy up by targeting the homepage as well as the major hubs and even specific product/business listing pages with authoritative backlinks.
These deep level pages absolutely need good content, and the more the merrier is the mindset you should take when approaching this issue. But before making a massive investment into content development, make sure you implement all the other tips and suggestions in this post first, and wait anywhere from a few weeks to a few months to actively monitor how your site is responding in terms of improved crawlability and indexability.
When monitoring crawlability success (cache:), keep in mind that cache dates of under 1 week are very strong, between 1 " 2 weeks is still very good, but once you start getting in the 3 " 4 week cache date range theres room for improvement, and anything beyond that you might want to pick up the pace on your link building efforts (assuming the site is aged, has a decent backlink profile, etc.). Make sure you also actively monitor your sites indexed pages (site:) and the site traffic to continuously see how your site is reacting to your efforts. By implementing the tips in this post you should benefit from improved traffic as a result of increased crawlability and indexability so keep that in mind and be sure to take benchmarks and actively monitor all of your efforts!