An important thing to remember, even when you are customer facing type of PM, is that one valid answer to a question is ‘No’.
If you say ‘yes’ to every request, every desire, every whim of the customer, you will certainly have a happy customer… …for a while. But when it gets closer to the deadline and you still have a stack of work to do, or you have run out of budget, or you now have every resource in the company working on your project for 18 hour days and weekends, they will start to lose faith in you. And that’s when all those favours you did them and all that wonderful helpfulness and positivity will be forgotten and the knives will be out.
You don’t necessarily need to be nasty about it, in fact in many cases you won’t even need to say the word itself. Just make sure everything you agree to do, will help you achieve the deliverables. And in all cases, make sure you use your big pad of Change Control notes.
But, when you can turn it round and make the person asking the question say ‘No’ for you, then you know you are good at your craft.
Tip: explain to them how much you would love to do what they are asking. Explain how, if it were up to you, you would have no hesitation in doing it. Then when they see how on their side you are, point out to them all the negatives on doing it, such as delays elsewhere in the project, lack of resources, extra cost, etc. If you are good enough, eventually they will see the error of their ways and back down, acknowledging of course how good a job you are doing.