• Links:
All major search engines crawl the web through link structures. If a site has a good link structure starting broadly from the top and going down into the category and sub category level, with all the money pages three to four clicks away from the home page, the bots would find crawling the site a lot easy. Placing a sitemap on the home page further assists the bots to find all the content on the site.

• Content Freshness and Updates:
This is one of the best ways to keep the bots coming back to your site regularly. It is vital to have fresh content updated regularly on a site. A blog will go a long way in achieving this. To a googlebot, new content is a sign of attaching more importance to the site by visiting it more often.
There is a Query Deserves Freshness (QDF) component in Google's algorithm that awards sites with updated content (news sites for example) that invites the bots back to the site for repeated crawling and indexing.

• Feeds:
If a site has a regularly updated blog or fresh articles posted on it at regular intervals, it would be ideal to have a feed and export it. Google Blog Search and feed tracking help in increasing the crawl activity. When a new post or article is published on the site, the search engine is pinged to let it know that the content has been updated.

• Importance of Domain:
A powerful domain that has good quality links coming in from diverse trustworthy domains is very important and it affects both the crawl rate and indexing of the site that resides on that domain.

• Technical Factors:
A site can have spider traps in the form of linking structures that have infinite loop system. The crawling can be interrupted by broken links. The problem of duplicate content with same content found on multiple URLs due to use of a CMS is also possible. All these factors inhibit the capacity of a bot to crawl the site exhaustively.

• Increase the Crawl Rate in Google Webmaster Tools:
If you login to the Google webmaster tools, there is a provision to increase the spider's crawl rate. It is a small consolation if the site is affected by problems listed above. On its own, it cannot influence the crawl rate to any extent.