My supervisor (also a developer) and I have a running joke about writing a book called "Managing From Beneath: Subversively Guiding Management to the Right Decision" and including a number of "techniques" we've developed for helping those who make the decisions to make the right ones. So far, we've got (cynicism warning!):

• BIC It! BIC stands for "Bury In Committee." When a bad idea comes up that someone wants to champion, we try to get it deferred to a committee for input. Typically it will either get killed outright (especially if other members of the committee are competing for you as a resource), or it will be hung up long enough that the proponent forgets about it.
• Smart, Stupid, or Expensive? When someone gets a visionary idea, offer them three ways to do it: a smart way, a stupid way, and an expensive way. The hope is that you've at least got a 2/3 shot of not having to do it the way that makes a piece of your soul die.
• All-Pro. It's a preemptive pro/con list in which you get into the mind of the (pr)opponent and think what would be cons against doing it your way. Twist them into pros and present them in your pro list before they have a chance to present them as cons.
• Dependicitis. Link pending decisions together, ideally with the proponent's pet project as the final link in the chain. Use this leverage to force action on those that have been put off.
• Preemptive Acceptance. Sometimes it's clear that management is going to go a particular direction regardless of advice to the contrary, and it's time to make the best of it. Take the opportunity to get something else you need, though. Approach the sponsor out of the blue and take the first step: "You know, I've been thinking about it, and while it's not the route I would advise, as long as we can get the schedule and budget for Project Awesome loosened up, I can work some magic to make your project fly."

So ... what techniques have you come up with to try to head off the problem projects or make the best of what may come?