If your working with Sandcastle, or just writing well documented sourcecode , then your familiar with Xml Comments. If not, then I would suggest starting to learn. You can get some great information here.
If your already using Xml Comments, I would recommend taking a few minutes and trying out GhostDoc from Roland Weigelt.
According to the GhostDoc site, the product is described as GhostDoc is a free add-in for Visual Studio that automatically generates XML documentation comments for C#. Either by using existing documentation inheritedfrom base classes or implemented interfaces, or by deducing comments fromname and type of e.g. methods, properties or parameters.
GhostDoc allows for automation of the generation of the Xml comment syntax, and also attempts to complete the comment itself, allowing you to skip most of the redundant entry that drives so many developers away from proper documentation.
You can get more information on GhostDoc, or download the install for Visual Studio here:
I use it, it's a great tool, and it's free.