#11.   Partition large projects into 3 month phases, that is the longest period you can plan for without the chance of significant change to priorities, resourcing, etc.

I was lucky to learn this early in the 90's as Project Management was getting a higher profile, accompanied by the increased use of Microsoft Project. Other PM tools were in use, but usually in limited numbers; MS Project, on the other hand, was readily available and budding Project Managers thought they could now plan the whole world for months or years in advance.  What actually happened, however, was constant re-planning as the reality of business change and resource turnover always took their toll. As Napoleon said, "A plan is only good until the battle is joined." After that, one must adapt to the changes that will always come.

My own experience was on a large project that was broken into a dozen pieces, which were planned separately to a target 18 months away, at which point I was asked to integrate them into one plan while resolving resource conflicts. First thing we found was that MS Project of that time crashed when you reached around a thousand tasks.

So, it was around then that some IBM PM consultants were brought on to sort out this mess, and where I first heard the above principle. Yes, you need a plan to get started and to control a project over time. You can even sketch out a plan out over many months to see and communicate the big picture; but, do not commit to any target date over 3 months away, odds are you will miss it. This also means you should do the detailed planning only to the next target date, meaning the full WBS and resource assignment.  (At the other end of the detail level, the IBMers also recommended that the shortest task in your WBS should be 2 weeks, no less, otherwise you are micro-managing.)

Once you have a detailed plan for three months, and a high-level plan for the rest of the project, you can add more detail to further target dates as the project progresses, as each interim target is reached or on a rolling month-by-month basis. The level of change in any 3 month period will be manageable, much more than over the whole project.

There is a more recent corollary to the three month principle; the age of the mega-project should be over by now.  Any IS project that takes many months or years to deliver the system is destined to fail. Yes, large systems are still needed, but break them into pieces that can be delivered, ideally about every 3 months; longer than that and you start to slide back to the mega-project approach, while shorter than that will not produce enough of the system to be worth delivering to the business. All business works in 3 month quarter cycles anyway, IT should too.