In the first year the tax bill usually arrives long after the money has been spent. The advance payments for the second year follow right behind it, and one payment quickly becomes two.

A simple rule helps. Put a third of every paid invoice into a separate account before you do anything else with it. That is often more than needed, but whatever is left belongs to you at the end of the year.

If you charge VAT, set it apart as well. It was never your money, it only passes through your account. A second sub account with that name makes it visible every day.

From the second year on it pays to look at the real numbers. With clean books the reserve can usually drop to a quarter, without unpleasant surprises.