The single most important part of a project is a shared, detailed, clearly-understood and agreed-upon description of what it is that you’re trying to end up with.
Most projects fail because of some minor detail, glossed over in the planning stages, which ends up being absolutely critical. In other words, most projects fail because of imperfect communication.
By definition, a project is anything you do once: it’s not an ongoing process, like cooking dinner every night, or doing the laundry. It has a beginning, a middle, and an end. It’s more-or-less one-time thing: building a deck, or planning a wedding, or buying a house (even if you do it more than once, it’s likely to be different each time).
A project plan does not have to be a complex and dependency-ridden mishmash of tasks and goals and deadlines. It’s just a way to think about what has to be done, how to organize it, and how to tell when you’re finished.
Other than consensus, you really need a project sponsor. This person wants the project to get done, and will provide the political impetus to keep it moving past obstacles. Just because you’re “in charge” doesn’t mean you have the power to do anything.
That’s another big reason projects fail.
There are many, many specialized tools for managing projects and so many different philosophies and styles that you can easily get caught up in the process of figuring out a process. Bottom line: figure out a way to outline what needs to be done, that allows you to see what’s been done so far and what hasn’t, and make it your own.
Final thought: scope creep keeps projects from ever being finished. If the scope starts to creep, try to nail it down by suggesting that any additional features can be added the next time through…which works a lot better for software than skyscrapers.