Scenario writing. What scenarios might cause the proposal to fail to produce the desired outcome. Be realistic. And yet, let your imagination run a little, so that you have a good chance of thinking of the most dangerous possibilities. In particular, think about the dangers of the implementation process, political and otherwise.
The other-guy’s-shoes heuristic. Image yourself in the other guy’s shoes. Say to yourself, “If I were X, how would I act?” And then proceed to crawl into X’s mind and play out, in your own mind, what X might do. Do this systematically for each of the important stakeholders or other affected parties. The value of doing this that you will discover them to be adapting in surprising ways to the new policy situation you might be creating, and with results that might cause trouble for your policy design.