Just make sure that your camera is a recording some kind of audio track even if it is from a crappy internal mic and then use a program like Pluraleyes to sync them up. http://www.redgiant.com/products/all/pluraleyes/
Plural eyes will automatically sync the audio from the camera with the audio from a separate source. Also Premiere Pro CC and Final Cut Pro X have a similar function, though I believe Pluraleyes works the best. I think that this would be your best solution because you will only have to remember to press record on the camera and separate recorder, the rest of the work will be done in post.