anything thats smelly and can be liquidized, herring works well, if you can pre bait, do so, are there any areas that have a normal light left on all night?, as baiting in these areas, the rats will already be used to the light, no lamp/filter is then needed in those areas
make sure you have comfortable shooting areas to sit, bales of hay/straw etc, as if its cold/hard/crampted, you prone to fidget/move more, making niose