Don't Involve the PM from the Start Of the Project
Think about a typical sales cycle for a minute. The account manager, salesman, business development manager (or whatever they are called in your neck of the woods) sells a wonderful solution to a customer. Statements of Work are drawn up, contracts are signed, and the project starts. If you're the project manager, there is a good chance that you enter the engagement at project initiation. The problem is that there is a lot of water under the bridge between that first meeting and project kickoff.
If the project manager isn't involved from the start, they miss out on crucialand often unwrittencommunication. The project manager doesn't have a firm idea of who are the players involvedthe real power sources and agendas.
I've been bitten by this "late arrival" problem a few times. Typically, I get called in after the project falters at the start or is simply running off-track. In one case, we were implementing Microsoft's Active Directory. For a number of reasons, we were not getting any traction and the project was stalled. Mr Fixit (that's me) was called in as the project manager. I wasn't involved in the sales cycle and had no direct contact with the project sponsor until after we had started.
After a few tweaks here and there, the project seemed to be back on track. We had completed scoping and requirements documents. I then began to get a nagging feeling that the client and our engineers were not on the same page. I called a meeting with all stakeholders, in which I asked the client to state explicitly what they thought we were doing. Sure enough, even if documents had been signed, meetings convened, and whiteboards filled, we completely missed what the customer wanted.
I traced the problem back to the initial meeting with the sponsor. His direction to us on the project and the vision I was handed were quite different. If I were involved at the start, I would have picked up both the written and verbal communication. In this case, we wasted three weeks; you might see your project drift over the edge with a problem like this.
Bottom line: The project manager has to be involved on the project as soon as possible. The alternative is miscommunication, trust breakdown, and project derailment.