Some good idea's have been submitted, I will have to take stock of my bag and upgrade it a bit.

A tip for you all, if you get two large bin bags, you can cut a hole in the bottom of one and two holes in the side, these are for the head and arms.  Then with the other one, you cut all the way across the bottom and use this like a skirt, pull it up to your waist then tie the slack(assuming you are not too round, around the waist,lol) into a not, you then have a water proof cover.  Must make sure the top bag is outside the waist band, so then the rain just runs down to the bottom.  I learned this tip from a refuse collector, emptying the bins.  Great in an emergency.