Compartmentalizing your plan by developing planning modules allows you to attack the plan in parts, yet still maintain a cohesive plan. I have found that developing an annual plan made up of quarterly targets - thus becoming a rolling quarterly forecast financial model - allows for a cohesive structure along with the nimbleness to react to market conditions. At the end of each quarter, a true-up process to align results to annual targets needs to be re-forecast and adjustments made.
Each project initiative should have a corresponding project plan that monitors whether it will be completed on-time and on-budget. The importance of the detailed project plan is to accomplish the following: a) identify all the steps to be completed; b) establish a realistic timeline for each step; c) identify and allocate the necessary resources for accomplishing the initiative; d) ensure that the initiative has been vetted for departmental inter-dependencies and potential conflicts; and e) ensure that the initiative is in alignment with the overall strategic plan.