One way is to add a standard using directives to common templates.
You do not specify which version of visual studio you are using, but in VS2010 you can find the templates in:
C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE\ItemTemplates\CSharp\Code\1033\Class.zip
Modify the Class.cs file in zip to include the ones you need by default.
The only drawback to this is that it is designed for each machine - so you will need to do this on every development computer, although I suppose you could run it using Active Directory (this is a question for serverfault.com).
I believe that command templates can also be customized - but this is not what I tried. Further information is available here:
http://msdn.microsoft.com/en-us/magazine/cc188697.aspx
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