What is the most important thing to keep in mind when preparing a WBS?

When preparing a WBS you should keep in mind the importance of all the documentation about scope and requirements of the project. This is very helpful for the project manager during the decomposition porcess.

While preparing WBS it is important to divide the scope of work involved in a project in managable tasks. The better you can manage a task the better will be control of the project and therefore more chances to deliver a project successfully. 

Team buy in accountability. The project manager need to involve all the team that are going to run the project activities.

A work breakdown structure (WBS) is a logically structured hierarchical breakdown of the Define project scope by areas and sub-areas.

Describe by facilities and sub-facilities, systems and sub-systems and units of property (assets or deliverables).

It has to describes what constitutes the project rather than how or when the deliverables are produced; the latter being described by the project execution plan and schedule.

The WBS evolves through the project stages and provides:

• a deliverables-oriented map of the project scope and battery limits
• key input for the development of the schedule and the capital cost estimate
• a framework for defining, controlling and reporting of project work
• a means for establishing the fixed assets register.

The WBS divides the project scope into the following, which are designated as WBS levels:

• Level1 – Area and facility
• Level2 – Sub-areas and sub-facilities
• Level3 – Systems and sub-systems
• Level4 – Asset
• Level5 – Commodity.

Krishna sahib explained everything. I will add that every work needs many things to complete but it should be complete within specified duration. Time period consideration will support project completion on time. 

Scope statement and enterprise environmental factor

As others have stated, when preparing  a WBS,be aware of:

- The tasks/projects and its sequence with other departments and resources;

- Timeline and responsibilities

- Extra cost