Here's a little tip based on a rather irritating experience!

I took delivery of a cheap (£20) lavalier mic (that's a tie clip mic for those that aren't into their sound yet!) called an ATR-35s. OK, it may not be the best 'personal' mic in the world, but at that sort of cost, does it matter? No, in fact, I was pleasantly surprised at the quality of sound it achieved recording direct into the EOS 5D Mark II, even with Auto Gain Control playing its tricks!

Anyway, this lavalier mic is on a looooong cable - like 3m or more. Again, this was a good thing becasue it allowed me to clip the mic to the subject and have the cable run away, back to the camera, and still hidden. 

Anyway, the problem came with the on-off switch. It's located on the side of a little battery compartment that you can clip to the belt of the subject. All was going well until the subject re-positioned the unit with the on-off switch and somehow knocked the switch off. Cue no sound recording for the next 5 or 6 clips and no knowledge by me as there are no audio level meters showing on the camera during recording. 

So how should you avoid it? Two steps: 1. Gaffer tape over the switch once set so it can't be knocked
2. Check the sound recording after every couple of clips to make sure all is still working properly. 

If at some stage we get the abilit to have headphones plugged into the side of the camera to monitor audio as it's being recorded, we'll be home free, but for now, it's a case of pay attention and make sure everything is right each time you get to record.