Set roles and goals early.
It may seem silly and basic, but setting roles and goals early will help out immensely as the project progresses. Specify who is doing what (research? writing? presenting?), with as much detail as possible and with dates and deadlines when appropriate. After all, knowing that one of your group members is going to complete part of the paper's research won't do any good if he completes it after the project due date.
Allow a time cushion at the end of your schedule.
Let's say the project is due on the 10th of the month. Aim to have everything done by the 5th or the 7th, just to be safe. After all, life happens: people get sick, files get lost, group members flake. Allowing for a little cushion will help prevent major stress (and a possible catastrophe) on the actual due date.
Arrange for periodic check-ins and updates before putting the final project together.
You may be working your you-know-what off to finish your part of the project, but not everyone may be as diligent. Arrange to meet as a group every other week to update each other, discuss how the project is going, or even just work on things together. This way, everyone will know the group, as a whole, is on track before it becomes too late to fix the problem.
Allow time for someone to go over the final project.
With so many people working on a project, things can often seem disconnected or confusing. Check in with a campus writing center, another group, your professor, or anyone else who may be helpful to review your final project before you turn it in. An extra set of eyes can be invaluable for a big project that will have an impact on so many people's grades.
Know that it's OK to talk to your professor if not everyone is pitching in.
One negative aspect of doing group projects is the possibility that one member (or more!) is not pitching in to help the rest of the group. Although you may feel awkward about doing so, know that it's OK to check in with your professor about what's happening (or not happening). You can do this midway through the project or at the end. Most professors will want to know and, if you check in midway through the project, they might be able to give you some advice about how to move forward.