I would normally do this, but I will try to explain keyword cache. I link out a lot more than most and I link to mostly .govs in my field. But in practice you can probably do this with any link system. I use their 'search terms' and sort of 'pick up on any thing relevant' THe system in WPW would not allow me show the list as it too large to demonstrate so I put it in my forum.
The following list is just part of my overall strategy. I did try PM'ing Peter with it, but it was too large for that too.
Keyword Cache
In all I have around 6,000 differing search terms that find me on page one or two of Google. This avoids using PPC or any form of advertising. Any questions, fire away.