2. Scope. Define scope well. Get your sponsor approval for scope changes, making sure the sponsor understands any schedule, budget or other impact to the project.In most projects what will be done can be ambiguous. The scope should remove that ambiguity; particularly by saying what will not be done that might be expected to be part of the project.
This can occur at interfaces with other activities (e.g. activities of suppliers, customers, the sponsor and other projects within the current program or portfolio).
Interfaces are a special problem because the boundary is where your project's expenditure should stop, or clarity created as to the process whereby the boundary can be crossed.
Change and configuration need to be pointedly managed to contain scope and ensure that the investment will produce the business results sought. Relax on your first change request from the sponsor's bright eyed young star, and you may have changed from the high road to the low road. You will wear it and not him or her.
Rules and processes enforced by the PM for change acceptance and budget and delivery effect need to be set. Additionally the effect on overall outturn performance and high value requirements needs to be advised to the sponsor: does he want THIS project, or some other project that will do some other thing?