I have a cunning plan..

Building your project plan is the most critical step in any project. Your plan is the basic way of measuring the progress of the project - and if you can't measure it you can't manage it. If your project is to be successful it must start with a solid plan. Much of comedy of BlackAdders sidekick and servant Baldrick lies in the unachievability of his "cunning plans" and are there lessons to be learned from the succinct communication of his plans?
But how can we access the quality of the project plan before execution - or before it goes pear-shaped? Here are my "top three"

• Completeness. Has everything been included? The best way to validate is through product based planning. You take the deliverable product and simply break it down into its component parts and assemblies (where there is some effort to put the parts together). So when planning a conference one area will be preparation of conference documentation - made up of component handouts for each session with the extra item of assembling the handouts into the conference binder. There should be activities corresponding to each component part and assembly.
• Accuracy. As well as requiring a reliable basis for estimating the effort required for each task the accuracy of the interdependance of tasks and resources is critical. So a simple plan (below) to code and test a simple client-server not only requires the accurate estimation of each task - overrun of any task will cause a delay to the whole project -  but also the accurate scheduling of resources. The resource to perform task 2 needs to be available immediately task 1 completes to meet the schedule. Overruning task 1 will waste the time for the resources committed for task 2 - which may also be the same resources for coding in task 3? If the complexity of the data needed is underestimated that will extend not just the database setup but the middle tier, the user interface and testing. 

• Socialising the plan. Most importantly the plan needs to be checked and committed to by everyone involved in the project. Too often the gantt chart will be produced by the PM - probably in Microsoft Project - and emailed out. However how are responses expected? Is there any narrative? Is there a list of assumptions used? Does everyone has the skills to use MS Project or even the software? So the ownership of the plan - and more importantly the committment to the plan and its accuracy- remains with the PM. Any mistakes remain the PMs problem. "I don't understand how the PM estimated this task so if I take longer then it's the PMs fault."
  To get commitment and belief in the project plan it needs to be "socialised" and fully communicated to get ownership by everyone involved. This communication needs innovative approaches - See Geof Lorys great "Lego My Schedule" and John Estrellas "How to create visual project timelines". But more importantly don't just depend on gantt charts; use narratives, brainstorming, risk workshops, feedback surveys or any way you can to get a true two way dialogue going to build trust and group committment.