The Canon 5D is a wonderful camera for shooting HD video footage, but one thing it isn't great at— sound. In order to get great quality audio, you're going to have to use an external sound recorder. If you want professional grade sound, you have to use professional equipment. You need to make sure your sound recorder has a sampling rate of 96 KHz at 16 bit resolution or better.
Also, make sure you have a windscreen for over the microphone grill, because it helps eliminate nasty unwanted sounds when outdoors.
But there's one problem with using a professional sound record with your Canon 5D or 7D camera. You won't be able to connect it without having a passive attenuator. In this video, Ritwika teaches how to get ultimate quality audio for your Canon 5D and 7D videos by using an external recorder and making your own passive attenuator cable.
* The recorder used in this video is the Sony PCM-D50 Portable Linear PCM Recorder.