Always Plan.  You can use anything to plan, Microsoft Project, Excel or even a napkin.  Several tricks that we use and reasons to plan are:

When begining a project it is important to plan out the major phases because lining up resources can take time.  Even if a specific engineering task is months away, start soliciting help because getting your project lined up with the availability of experts can take some time.

Development plans can change.  Unforeseen technological obstacles can emerge in the investigation process.  It is important to build in some flexibility to the plan to account for these unforeseen road blocks.

Planning minimizes risk.  The stage gate process was created to accomplish this.  Setup milestones in your plan that allow you to step back and evaluate progress, direction and the future.  We use several milestones, complete Industrial Design sketchups, CAD renderings, Form Prototype, Functional Prototype, Complete Prototype etc all the way up to Production Run.

The only warning that I will provide is ‘don’t plan on planning’.  Do it right now.