To get team member commitment to a schedule, the team has to have been involved in the planning process—not handed a schedule that someone else created—and they have to agree that the resulting plan is reasonable. The following approaches can help ensure you get real commitment:
Involve the team in the scheduling process. See our item "Who creates the Project Plan?" for a discussion of how the team should be involved in creating the project plan and why. Specifically, individual team members must get to develop the estimates for their own work. Why should they sign up to tasks that someone else has estimated for them? At the very least, if someone else drafts some estimates (whether their functional manager or the project manager or even a respected colleague), the team member still must get to review and adjust those estimates.
Conduct a team review of the schedule as it starts to come together, and definitely as the team thinks they're almost done planning. Does each person agree with what has been included in the schedule for their work? A good tool to use for a schedule review is the Schedule Checklist, which contains items to ensure your schedule includes all project work, such as cross-functional activities, testing, and more. It also provides a few guidelines for getting the team through this critical phase.
Ask leading questions. Typically, lack of commitment to the overall project plan—including the schedule dates—is a big indicator of risk. Ask why they don't agree with the plan, which elements cause them concern, and what would they propose as an alternative. Listen to their concerns. Are they avoiding commitment because there are risks on their minds that haven't been accounted for? What do they recommend to increase their confidence and gain their commitment to the plan? What impact does their recommendation have; does it violate a schedule or resource objective?
In short, the way to get commitment is to involve individual team members in all the steps of the planning process, review the schedule to look for holes, ask questions to ferret out the reasons behind any apparent lack of commitment, and listen and address those issues!