I am currently working with a client that, like most clients, is insisting on complete documentation of whatever I do while on-site. I find the most-productive way for me to do this is to use desktop Wiki software on my laptop as my documentation tool.
The desktop Wiki software allows me to quickly enter notes as I work on items, dynamically creating an intuitive structure for the information. For less formal engagements, the printouts from the Wiki itself are sufficient. For more formal engagements, the desktop Wiki helps me organize my thoughts and I can then easily transfer my Wiki text into the formal documentation format required by the client.
If the client does not have formal documentation requirements, I have been successful in getting the client to adopt a Wiki as their technical documentation repository. Once they see how easy it is to enter and maintain the information, find it via the full-text search, and access it as a web-site from anywhere (server version), they are sold on it. I point out that for maximum benefit, the Wiki should be secured for access only by technical staff and one or more separate Wiki sites should be set up if it is to be used outside of technical staff.
If you are interested in trying out a Wiki, I have been very happy with the ScrewTurn Wiki. It is written in .Net, has both a Windows desktop version and an IIS version, and is available at no charge under the GPL v2 licence. It has a nice graphical toolbar in edit mode plus learning the formatting tags is quick and easy. I find it very intuitive. Online hosting is available for a fee if desired.
If you have not yet tried using a Wiki to collect and organize your documentation, I recommend that you do so. You will be amazed at how much documentation you will actually do real-time once it becomes so easy with the Wiki. Since finding things with the full-text search feature is so easy and fast, you will become self-motivated to keep it updated as you work, just as a future reference.
I also find it very helpful to use the Wiki to organize my findings as I work through a difficult issue. This helps me arrive at a solution faster, and the process of how I arrived at the solution is documented as a bonus.
Hope that helps.