What helped me the best is focusing on problems, not on solutions. I don’t ask myself anymore how I can make a player to do X. More like I try to make up a problem where all easy solutions fail.
Then I think about how I would tackle this problem. And if I still can think of only one solution the contract might not work out.
See my submission for this month’s batch:
The problem I set up is: You have only one detonator for two explosives of the same kind. Add a different disguise restriction for both targets and there you go. You can’t do what people always did with explosive device contracts: Press the trigger at the exit. At least one of the two kills have to be done way before that.
When I thought about the solutions, I found a good few of them. It already doubles the amount of them because of the symmetry. Whatever the player might come up with target A might be viable for B too.
Both explosives are on the map. I don’t even need to limit the loadout.