Hi Eva
The answer really depends on what modules you are using. If it's simply AP/AR/GL, then the answer is to set up new GL accounts every year (e.g. 40001 would be sales in year 2001, 40002 sales in 2002, etc - you can repeat every ten years).
If you are using OrderEntry or Invoices for billing, then the answer isn't so straightforward. If it's Invoices, then JobCost is a reasonable solution, as Andrew suggests. If it's OrderEntry, then I'd have to think further.
In any case, I don't think departments is a good way to go.
Steve