A number of DSLR's offer the opportunity of connecting an external microphone. You should take advantage of that in order to:
Firstly: The camera's own microphone(s) likely has an socalled Omnidirectional characteristic. That is: They collect sounds from all around the mic. Illustrated:
A microphone with Cardioid characteristic will instead focus on sounds coming only from in front of the microphone. Thus you focus on what the mic is pointing at thereby minimizing background noice. Illustrated:
You could go out and buy a stereo microphone suited for connecting directly to a camera like one of the popular (and a bit pricy) Røde models. I happened to own the little Zoom H1 sound recorder. The recorder has a 3.5 mm line out meant for headphones. The DSLR (in my case a Nikon D7000) has a 3.5 mm highly sensitive mic input jack. So in order to level the signal from the recorder I had to either:
In the first place I tried fiddling, but found that the signal ended up being too weak. Then I ordered the cable from Pinknoise-Systems in the UK - at 1.2m in order to keep the mic away from camera noise. Be aware that they charge £15 for the delivery outside of England.
But I also recorded the sound directly to the Zoom ZH1. And this signal was very fine. Only trouble was how to syncronize it with the video? Relying on my cable connection, I hadn't followed this advice: Start the videorecording and the soundrecorder. Then place yourself in front of the camera and clap your hands two times. Afterwards you sync the video and audio by looking and listening to the hand claps.
I had another trouble with one of my clips: The soundrecorder had fallen to the floor, when I moved myself, and unlyckily I managed to stop the videorecording while putting the soundrecorder back in place. So I had a gap in my videorecording. Another sync challenge.
I solved all my syncing problems using a 30 day trial version (fully functional) of Pluraleyes3! Faster than lightning this amazing program syncs audio and video like a charm. Works for OS-X and Windows, with plugin for a number of popular video editors or as a standalone program, so you can import the resulting two .mov and .wav files into any video editing program. Only demand is that the input audio must be in .wav or .aiff format. That can be easily fixed by a free converter program like Free MP3 WMA Converter (which also makes .wav files). AND that you simultaneously record sound on the DSLR, because Pluraleyes works by comparing the audio from the DSLR with the audio from the Zoom - and deleting the DSLR audio afterwards.
This is how Pluraleyes3 (standalone) looks after syncing an ordinary recording of audio and videosound:
And here is a demonstration on working with Pluraleyes in FinalCut Pro (by plugin) syncing video from a Canon 5D and audio from a Zoom ZH4N:
A huge advantage of using a separate sound recorder is that you are totally free of cables between DSLR and recorder (i.e. microphone). This means:
Finally I'll show you my recording that had a gap in the video compared with the Zoom H1 audio fixed by Pluraleyes. In the editing process I managed to stich the two parts of "On Green Dolphin Street" together while the drummer was doing solos. You'll hardly notice the stiching that goes on at about 3:05.