E.g.:
[DllImport("coredll", EntryPoint="GetDC", SetLastError=true)]
private static extern IntPtr GetDCCE(IntPtr hWnd);
http://www.pinvoke.net/
They have an Add-in to Visual Studio 2003 and 2005, to make the insertion of PInvoke signatures an easy and fast operation.
You can download it from their page. It's FREE :)
Best regards,
Gabriel Minárik