Flexibility
If possible, enter numbers only once.
Enter constants in a cell and use absolute cell addressing to reference its value. This will allow you to change the value once and without the need to re-enter the formula on the spreadsheet.
Example: HST rate, interest rate, tax rate.
Layout
Minimize the number of empty columns. Adjust the column widths to add the spacing you need
Correctness
Enter your formulas once, copy and paste as required
Make sure your numbers are reasonable.
Remember the rules of BEDMAS
Common errors
Entering a percent incorrectly.
Misplaced or forgotten brackets in formulae.
Not including all values required in the formula.