I play a big creature pet spellbook, so my Beastmaster is constantly putting nullify, reverse attack, block, and reverse magic on his pet (usually a Steelclaw Grizzly or Highland Unicorn).
Occasionally, when all of those resources have been used up, I'll throw down a decoy, making my opponent think that I'm just putting another one of my buffs on it and making him reluctant to attack.
I think Decoy is best used (like any other bluff), when you can set up your opponents expectations. If you put a hellfire trap in two zones and they both go off. Then next time, you can just play a decoy and a hellfire trap, you've conditioned your opponent to believe that both of them must be hellfire traps.
After that, once it's revealed that one of them was a decoy - he'll be more likely to walk into zones with facedown enchantments (afterall, he's thinking, some of them are probably decoys), that's when you can double down with two more hellfire traps. Etc, etc.