How to keep my manbag in place If I'm going out in the evening and don't want to carry a big pannier bag or rucksack, I keep my lights and whatever else in a small 'man-bag'. It's like a smaller version of a messenger bag: about 20 x 15 cm on a shoulder strap. I try to keep it towards my back rather than under my arm, to stop it swinging around, but invariably it slides out (especially on right-hand bends) and towards my front. If I leave it, it dangles under my chest and rubs on my knees as I pedal.

What can I do to keep it in place? I've tried shortening and lengthening the strap, and I've used it nearly empty or quite heavy, and it doesn't seem to make a difference.
At the beginning of this video:

http://vimeo.com/41982043

it looks like this bike messenger has an extra strap for his bag that comes up under his left arm to stabilize his load.  Looks pretty neat, like it can be easily unsnapped to get the normal messenger bag access, and possibly be DIY-able.