Here's the best Project Management tip I've learned over the years:
If you ask people to do two, three, or more things, they won't do any of them. Especially if they are volunteers, or are only part time on your project. But if you ask them to do ONE thing, and ask them when they can complete it, they'll almost always do it.
This is why daily stand up meetings are successful: what are you working on, what did you do yesterday, did you finish what you hoped to, and did you run into any road blocks that I can clear away.
This also has an impact on the size of the tasks you create in a project plan. In my opinion, they should be no smaller than a day's work, usually about a week's work, and no more than two week's work.
I implied earlier that having shared resources is often difficult. It provides them an excuse for not completing their tasks. If you have a critical project, make sure your key resources are dedicated to your project, and their other duties are handled by someone else.