The trinity, or three legged stool, for project management is well known to be scope, time, and budget. Contemporary wisdom argues that you must pick two of the three to focus on. The truth of it is scope controls everything, it might be better to label the three-legged stool as discipline, budget, and time. I would go further to say that the lynchpin of effective project management is D&D. I am not talking about the roleplaying game - in this case D&D stands for discipline and decision making. Let's revise the trinity to D&D, Budget, and Time.
The Project Management Institute (PMI) publishes a rather lengthy set of proceses and standards called the Project Management Body of Knowledge - setting the standard for proper project management. Passing the PMP exam is no simple feat, but that body of knowledge it seems gets thrown out the window as soon as the exam is over. Few organizations have a Project Management Office that follows PMBOK, or any well defined procedure for managing projects. Project managers and project management offices are treated as administrative assistants for projects in which they are sidelined, or they are handed the reigns of the project as the actual manager but with no real power, and sometimes not even invited to the meetings. Seldom is the project manager treated as the impartial arbiter, observer, and guide that is the purported purpose of project management.
Ultimately the fault in a failed project belongs to the leadership team of the organization. Bringing in a project manager but failing to empower and support them is a recipe for disaster. Assigning a PM is not an abdication of authority leadership must either A. be available, involved, make timely decisions, and be accountable to the project manager or B. assign the project manager or someone the delegated authority to act on their behalf with 100% support. Hiring a project manager or establishing a PMO office is not an excuse to sit out of the game. This is the decision part of the D&D leg.
Discipline speaks to many things in the project. The first area though needs to be backing up the decisions with discipline. Getting into a habit of making decisions that you have no intention of sticking with or knowing in advance are likely to change creates lethargy and apathy in the team. Constantly cancelling or rescheduling meetings does the same thing. To hit the time part of the trinity you have to fight inertia and ruthlessly strip away anything that slows momentum, including bad personal habits. Discipline involves having meetings mean something - don't create a meeting if one is not needed, always have an agenda. With respect to meetings be careful who you invite - if the person doesn't have the authority to make a decision don't bring them to the meeting. Decisions makers should be responsible for getting the opinion of those that report to them, and this should happen outside of the project meetings. Discipline speaks to communication as well - on a project all parties need to be disciplined about communication channels and biased against assumptions that knowledge is common. Discipline in these areas will lead to natural scope control, but when changes are floated up for the scope the burden should be on the requestor to prove need rather than the Project Manager having to defend the scope.
Bottom line(tldr):
- Discipline and Decision making are the determiners of project success.