While you are working on your outline you will most likely research your topic--especially to confirm the document you are about to write does not yet exist! Here are a few pointers that will keep you from pulling out your hair later:
Compile your resources as you research. It is almost guaranteed you will not remember where to find a critical piece of information when you need it most. It will help to bookmark important (and even not so important) pages as you go. Make sure the bookmark's title reflects why the page is important to you. If there are multiple key ideas in one page, you may want to bookmark the same page with different titles.
Assume your most important resource will disappear. The dreaded "Error 404: Page not found". Even if you have bookmarked a page it may not be there when you return to it. If a page contains a really critical piece of information: make a copy. You may do this by creating a text file with the title of the document, the author's name, the page's URL and the text of the page into a text file on your computer. You might also choose to "print" the file to a PDF (save as or convert to PDF format will capture the original URL on the page if you're using a smart browser).
Start your "Resources" page now. As you find pages of interest add them to a Resources document. You may do this by exporting your bookmarks or by keeping a separate text file with the Resources sorted by sub-category. A little effort now will save you a lot of time later.
There is more information about the DocBook markup of bibliographies in Section D.7.
Write down subject areas as you go. If you are card sorting you may find it particularly useful to write topic cards as you find pages that cover that specific topic. At the top of the card write the subject area. In the main area of the card write a few notes about what you might cover under this topic--include the titles of pages that contain important information. If a sub-topic gets too big you may want to divide it into multiple cards.
Separate generic information from version-specific information. A new version of the software that you describe might be released the day after you release your document. Other things, like where to download the software, won't change. Alternatively, you may choose to document old problems with specific software as a way of encouraging readers to upgrade to the latest version available: "Version X of the software is known for a specific bug. The bug was fixed as of Version Y."
Save all related emails. People will often have interesting insight into the problem that you are writing about. Any questions that are asked about your topic should be addressed in the final document. If you are writing about software make sure to ask people what system they are using. Add information in your document about which system configurations your instructions have been tested on. (Having lots of friends with moderately different configurations can be very beneficial!) All of these personal experiences can add greatly to your final documentation.