One perspective is that negotiation of schedule and resources is often nothing resembling estimation but is a political game. Ed Yourdon's book "Death March" has a chapter about Negotiation .

http://ptgmedia.pearsoncmg.com/images/013143635X/samplechapt...

There's a list of "negotiation games" that, sadly, you'll probably recognise. And a bit of more practical advice:

  >> The problem I have with “estimate” is that I understand
  >> the word to mean, “best assessment knowing what is known
  >> at this moment and what can reasonably be predicted about
  >> the future.” From experience, what I have seen happen is
  >> that once an estimate is published,it becomes a rock 
  >> solid commitment representing the maximum amount of
  >> resources needed to accomplish the project phase or the
  >> entire project.

  > Unfortunately, there’s an uglier aspect of the political
  > negotiation when you introduce the plus-or-minus
  > qualifier into your estimate: You’ll be accused of
  > uncertainty, wishy-washiness, weakness, or even
  > incompetence. [...] What senior management really wants 
  > is a firm commitment — a promise that the project will
  > be finished on a certain deadline, with a budget of a 
  > certain number of dollars, and a staff of a certain size.
  > This gives them the enormous luxury of (a) no longer
  > having to worry about the problem for the duration of the
  > project and (b) having a convenient scapegoat to blame if
  > the promise is broken.

  > Jim McCarthy, in his excellent book, Dynamics of Software
  > Development, suggests that the project manager needs to
  > confront this head-on and persuade the customers and/or
  > senior management that they need to share some of the
  > burden of uncertainty [around schedule, cost, resourcing]
  > that the entire project team will be living with on a
  > day-to-day basis. Thus, the project manager effectively
  > says to the customer or the senior management group,
  > “Look, I don’t know precisely when this project will
  > finish — but since I’m the project manager, I’m far more
  > likely than anyone else in the organization to figure it
  > out as soon as it can be figured out. I promise you that
  > once I know, I’ll tell you right away.”

  > Only a manager with a lot of self-confidence, and the
  > ability to walk away from the assignment, has the
  > chutzpah to say something like this in the politically
  > charged atmosphere of a death march project. The time to
  > say it is at the beginning of the project; after all, if
  > the customer and senior management do not respect your
  > ability as a project manager, and if they don’t realize
  > that you do have a better chance of knowing when the
  > project will finish than anyone else, then why are they
  > putting you in charge of the project in the first place?
  > Are you being set up as a scapegoat? Are you going to
  > be a “puppet manager,” with all the decisions being
  > made by other political manipulators in the
  > organization? If so, now is the time to get out!

  > Similarly, if you’re a lowly programmer on the project
  > team and you see political games like this, it may be a
  > strong indication that your project manager (a) doesn’t
  > have the confidence to believe in any estimate that he
  > puts forth, (b) doesn’t have the backbone to stand up
  > for himself and for the project team, and/or (c) has
  > gotten himself into a political situation where all the
  > key decisions will be made by people who are not
  > directly involved in the project. Again, this is a
  > strong indication that the project is doomed; and
  > before you get too deeply involved, it might be a
  > better idea to seek greener pastures.

Refusing to be pressured into emitting on-the-spot "estimates" that will be regarded as commitments is a useful life skill. Teach your new hires: http://www.dadhacker.com/blog/?p=2267