Preventing flies not just catching them

I have 3 in my garden (1/2 acre with loads of trees plus a beck at the side so thousands of flies normally) my redtops are mainly in the shade in the trees and like Richy said they can take a couple of weeks to work. Mine got about 3/4 full with flies then maggots appeared and broke it all down back to half full. They do smell when you are close to them and they are working, but boy the lack of flies in the garden and house is fantastic. Last year we were plagued with flies in the house (had 2 of the fly zapping things going, fly spray, swatters, window stickies and the old fashioned brown sticky tapes - yuk  ) but this year I think so far I have had a grand total of 4 flies