And recognize or communicate uncertainty. Too often I think that people like to brush uncertainty under the carpet, and then forget that their nice clean plan is just one of many possible outcomes.
That and understanding the relationship between risk, uncertainty and reward. If a project is not risky at all, there is probably no reward to doing it either. Risks contribute uncertainty to the schedule, so both the probability of those risks materializing and the time and monetary costs of mitigating those risks must be factored into the schedule and budget.