The plastic bag trick is to put them on over your socks and inside your shoes, for extra warmth in cold weather?
I had thought plastic bags were more for when its raining?
I rode for 7 hours in torrential rain (on the Alps to Ocean in New Zealand) 2 days ago - got soaked through under all my "waterproofs" including overboots- should have added a layer of plastic bags too! Yet it was strangely enjoyable - maybe it was just the thought that it's still better than the best day in the office ...