i was told to keep a book where I write down all my earning every week.  Then in the back of the book I have my expenses.  I split them into things like parking, vehicle, equipment, fuel etc and then I have columns  saying date, amount, type of expense, method of payment (cash or bank).  Then I put a number on every receipt I have and I put that number in the book next to the details of the payment.  It only takes half an hour every couple of months and saves me £150 every year