If you've exhausted all other methods in fixing this issue (which is usually addressed just by restarting Visual Studio), and still can't get IntelliSense to work, make sure the file you're working in is included in your project. Crazy, right?
Just right click the file and select "Include in Project". Of course, in order to view the file in the Solution Explorer tree, you would have to have "Show All Files" enabled. Therefore, this case is pretty obscure, but if it helps even one person I'll be happy. I'm going to grab another coffee now.