Mrsnec and Dolomites brought up good points.
Re Mrsnec's DH's brief (which sounds a nightmare) , part of the job is to keep going back to the project sponsor/s to get enough info to work with and also part of the project is to see if the project is actually going to be able to fly in the first place. Both very very tricky as you make a pain in the ass of yourself to the people who possibly hired you, and you could potentially have to deliver bad news or even do yourself out of a job, but better doing that than flogging a dead horse. That's where whatever natural charm you have comes into play. Being 'likeable' is key, also making yourself appear trustworthy so if you deliver bad news, backed up with clear facts you're more likely to be beleived, but I have seen lots of PMs in the firing line for things that are just not their fault, but that is definitely one of the downsides of the job.
Re jfdi, I have used the phrase (ithout the f) very very occassionally, when I'm exasperated, but probably not wise, though the thought goes round and round in my head dozens of times a day. It's to do with matrix management (useful phrase for job interviews) you have no direct authority over these people, they usually have their own bosses, so you have to dig into your mental toolbag and work out what you can do to make these people do what you want. Different approaches work with diferent people. My personal style is 'pleasantly persistant', it seems to work on most occassions. I did hear someone describe me as a Rottweiller, but as he's pretty co-operative and we seem to get on, I took it as a compliment 
oh, and one last tip, if you stroll over to someone's desk and they have a picture of their kid on there, compliment them on that. Talks about kids, in my experience, softens the hardest hearts, before you launch into the work stuff. Same applies to any other interests you may know they have.