First, find the thread you want.
Example, I want to link to KR's post in the Darkest Desires thread on page 36 in which he mentions Motherwar (towards the end of the page).
This is the Darkest Desires Part 2 thread:
http://www4.head-fi.org/forums/showt...threadid=21050
Second, find the post you want.
Third, view source.
Fourth, in the viewed source, find a keyword or something of the specific post to which you want to link.
Fifth, scroll back just a little ways, and find the "<a name=" HTML tag. That's your postid.
In the above example, it's "post366371".
Now, combine the two, separated by a pound-sign:
http://www4.head-fi.org/forums/showt...=36#post366371
All links to specific posts within Head-Fi will take this form. The first part, before the pound, will be an exact copy of the URL displayed in your browser when you are on the right page. The postid tells your browser to go to the right part of the page.