For my part I tend to write documentation as I write code :
I wrote a general (abstract) documentation, then I add more detailed parts.
(then I iterate the second phase until satisfied).

Note that you can do it

• In the same document
  Taking care to put the simple abstract part at the beginning and hiding as much as possible the too specifics details (in an APPENDIX for example)
• In several documents
  This allow you to specialize your documentation to the reader
  (Beccause a techie don't need the same informations as a commercial...)

The second way is my favourite way, I usually make a (very) small doc describing the product
(just to make the reader understand what we're talking about)
I then write a more detailed document with links to several other (specialized) docs (Appendix, user guide, maintenance guide, sysadmin's guide, developper's guide...).
document.

Last note: 'the quick install' and 'common troubleshooting' parts seems to me the most used part of a technical docs
(at least at my office), it may be worthy to write them with a special care...

UPDATE : The 'links/resources' part is also a must write to my mind...

UPDATE 2: As I realize that my answer may be far from practical, I'll add some common useful guidelines :

• Write a READABLE way
  Otherwise nobody will (be able to) use it...
• Adapt your text to your reader (the content AND the form)
• Don't forget your initial goal
  your technical doc shouldn't become a technical rant about how good the solution is...In the same vein in a commercial documentation, it's totally useless to explain your over-smart technical hack, speak about technological advance ;-)
• Use spell checkers (It's VERY important for some people (usually your bosses ;-))
• Use a portable format to allow other people to work on your documentation, convert it...

"Only Bad Coders Code Badly In Perl" (OBC2BIP)

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