Regularly review estimates, deadlines and milestones to check they are still on schedule. If necessary, update the plan with new estimates and tasks and ensure that remaining tasks are necessary and estimates are still valid. Circumstances can change during a project but question every change and always refer back to the original business requirement as a corss-reference.
Every project should have a contingency allowance for both time and budget as there is always a tendency to under-estimate tasks, particularly when under pressure to deliver as soon as possible. Indeed, in practise, many project managers are not given the luxury of estimating the total time required but are likely to be given a fixed end date and the project tasks have to be fitted into the time available.