One of the key indicators of project success is bringing it on-time and one of the classic challenges of project managers is that at least some of the people needed to deliver the project on-time do not report to the project manager.
Delia Cioffi and Randy Garner published the results of two studies in the Personality and Social Psychology Bulletin of February 1996 showing that by getting people to actively commit to an action they were more likely to do it. Analyses of the studies suggested that when people actively commit to something they will reason their commitment and strengthen their resolve to deliver on it. People are also less likely to refuse to commit when they are actively solicited because of social influence.
When you are planning activities for a project, ask the resources to supply the delivery date themselves and to confirm it. Do not rely on implied dates. Avoid suggesting dates. Prefer getting the resource to give the date themselves and confirm. If possible, have them put it in writing. The more active the act of committing to a date is, the more likely the resource will make the effort to deliver on-time.