I mentioned this elsewhere (I think) but we would pour a little gatorade on the floor of the canoe and every fly in/around the canoe would soon make it's way to to the puddle in search of a sugar high.  Once there were 50+ flies on the small puddle we could kill all of them with one good whack of a towel or our hat.  We just sponged out the carcasses and would repeat the process as needed throughout the day.