Identify what it is. The first thing is to identify the who, what, when, where, and how of the project.
Get it out of your head. Begin to write it down in a notebook or electronic file so you can get it out of your head. Your conscious mind is best used for focusing not for storage as David Allen will tell you.
Chunk it down. This is another way of looking at the who, what, when, where, and how. Don't let the monster take over. There are ways to break down any project into doable parts. Start to put your ideas into categories.
Prioritize. Keep asking yourself what is most important in the project and adjust things as you go.
Assign enough time. Often people don't allow enough time to complete a task. The rule of thumb is to add 50% more time than you think it will take. So if you think it will take an hour for a task, plan on an hour and a half.
Delegate. And, last but not least, delegate tasks to other people. There is always more that can done by someone else than you think. Take a moment to identify tasks and people who can help.