Integrate your financial modeling with your historic accounting system.  Your model format should closely resemble your monthly historical accounting management reporting.  This is important so you can maintain an "apples to apples" comparison of forecasted results with actual results.

Build version control and e-mail communications into your financial modeling program to facilitate what-if analysis and collaboration.

Document the sources of information and critical assumptions.  Be prepared to justify and defend your numbers.

Document your model with both user instructions and system documentation.

Include verification reports in your model which include comparative summaries of assumptions made and related results.  This will help you quickly find errors.

Update your financial model at least annually.

Present your model monthly for the first 12 months and annually for years 2 through 5.  Extending a financial forecast beyond a 5 year forecast period  is not generally considered useful.

The first twelve months of your financial forecast should be your annual budget.  Any changes from the annual budget, as adapted by the Board of Directors, should be documented.

Actual operating results should be compared at least quarterly to forecasted  results.  Written explanations of significant variations should included in the MD&A presented to the Board, investors and creditors.  Comparisons should be detailed and include sales by product line in units and dollars, head counts by department, and financial ratios.