Know your business and understand its relationships and dependencies.

Develop a sound understanding of basic accounting concepts.  Then be sure that your accounting systems are designed to give you the financial information you require to run your business.   Accurate historical accounting records are the key to preparing a meaningful an accurate financial forecast.

Understand the fixed and variable cost structure of your business.

Know what your product really costs.   For companies with a complex cost structure and many stages of production, implementation of standard costing systems may be necessary to accurately compute costs.

Develop some basic computer and spreadsheet skills.  Your financial model will probably be programmed by your accounting department, financial analyst, and perhaps outside consultants.  But as a tool, the financial model is most useful and powerful in the hands of the business' managers.