Off-page optimization is defined as any tactic used to optimized your site without actually modifying the code on the pages. Below are the most important. 

• Link Anchor Text - By far, the most important part of search engine optimization. Getting inbound links with the proper link text is vital. If you can, try and vary your anchor text as much as possible to make the links look as natural as possible. 
  
• Inbound Links from Varying IP's - Get as many as you can but make sure the links you get aren't largely based on one set of IP addresses. In other words, if you have a bunch of sites only link them together where it makes sense. Theme related non-reciprocated links seem to get the most value.
  
• Site Wide Inbound Links - MSN and Yahoo seem to reward inbound links from multiple pages of one site. Google will only regard the first and slowly give them less and less power, MSN and Yahoo seem to tally all of them equally.
  
• Directory Listings - Each search engine has a directory that they favor more. MSN - Zeal/Looksmart, Yahoo - Yahoo! Directory, Google - DMOZ/Google Directory. To keep all of your bases covered it's best to get listed in as many as possible as many times as possible.