Last year I built 4 Nuc Boxes from plans on this site. About the first of April I put them out in places where I had caught swarms previously. I have now caught 5 swarms with these boxes. I place 5 frames of drawn comb in them with some Lemongrass oil and it is an easy to get swarms since I can't be at all these places to catch a swarm when they fly.
I made a frame of 3/4" X 3/4" wood strips and covered them with 1/8" wire cloth with 2 screws to hold them in place [provides extra ventilation]. Just slip these covers over the opening and head for home with the bees. These are swarms that I wouldn't have caught if I hadn't used these bait boxes.
I use the lemongrass oil too. But the main "ingredient", you have already discovered. Location, location, location. http://www.beesource.com/ubb/smile.gif