Automatically add using directives to open C# files.
Include a file called .alwaysuse in the same folder as the *.csproj or *.sln file. (It doesn't need to be included in the solution.)
On each line of the file, add a namespace you want included in every C# file you open.
When you open any *.cs file, the specified namespaces will be added as using directives (if they're not already there.)