How to include other files to the output directory in C# upon build?

I have some library files needed for my application to work.
My application has a setup and deployment included.

I already know that in order for a library file to be added to the output directory of the application when installing, I just have to reference those libraries inside the .NET IDE before building... the only problem is that these libraries can't be referenced... So I need to be able to copy these libraries to the installation directory of my application... At the moment, I am copying these libraries manually...

Addendum

I also did try to add these library files as an Existing Item to my project and marked each library files' Copy to Output Directory to Copy if newer on their properties but still not getting the solution I want.

Update 1

Thanks for you help guys it helped me solve my problem, I managed to make the solutions you posted work except for one... @Matthew Watson's post.. I even managed to find a solution too so I wanted to share it with you also.

Heres what I did:

1. I opened the setup and deployment project in my application.
2. Under the Application Folder Tree, on it's right side, I right clicked..
3. then clicked Add..
4. then clicked File
5. and then browsed for the files I wanted to add to the installation directory
6. and click open.

But out of curiosity...I am still trying to make what @Matthew Watson posted work...Hope you can help me with this one guys. Thanks in advance

Update 2

I forgot to update this post yesterday, I already manage to make Matthew Watson's solution worked yesterday. Thank you again for all your help guys.

You can add files to your project and select their properties: "Build Action" as "Content" and "Copy to output directory" as "Copy Always" or Copy if Newer (the latter is preferable because otherwise the project rebuilds fully every time you build it).

Then those files will be copied to your output folder.

This is better than using a post build step because Visual Studio will know that the files are part of the project. (That affects things like ClickOnce applications which need to know what files to add to the clickonce data.)

You will also be more easily able to see which files are in the project because they will be listed with the source code files rather than hidden in a post-build step. And also Source Control can be used with them more easily.

Once you have added "Content" files to your project, you will be able to add them to a Visual Studio 2010 Setup and Deployment project as follows:

Go into your Setup project and add to your "Application Folder" output the Project Output called "Content Files". If you right-click the Content Files after adding them you can select "outputs" and see what it's going to copy.

Note that Setup and Deployment projects are NOT supported in Visual Studio 2012.

You can use Visual Studio Post Build Event - Copy to Relative Directory Location. Which are basically scripts that are executed on build of specified project.

So you can use it to copy binaries you need, before actually running your application.