D. Do Not Force Your Visitors To Register Before Accessing Pages
There are many sites where you have to register before you are able to gain access to certain pages. This is also a problem in that spiders (which are unable to create a username and password) will not be able to access these pages and therefore they will not be indexed.
E. Avoid cookies and Session IDs
In the same way as explained in D above, it is important to remember that the search engine spiders do not have cookies enabled and they cannot accept or pass session IDs properly. Thus it stands to reason that if you limit your site users ability to access certain pages or redirect based on a cookie setting or session ID then it is possible that the main search engines will not be able to efficiently crawl your content.
Overall then it is important to remember that in many ways 'simple is best' and factors such as HTML content and text links rather than graphic links will enhance not only the human visitors was also the search engine spiders experience of your web pages ensuring that these pages will be correctly and properly indexed by the main search engines. At the end of the day it doesn't make any sense to make life unnecessarily difficult for the search engine spiders and if you make their lives easier then you'll probably rewarded by much greater levels of traffic to your web site.