Dave,
When you start from scratch, you'll find plenty of 'bad customers', especially once familiarity begins to breed contempt.
Be firm, remind them of your original agreement and drop 'em if they try to shift the terms too much in their favour.
What happens is, over the years you'll get a better customer base, purely by dropping the dross.
Of course I still have the odd customer who tries it on, but it's rare it happens these days 'cos we get rid of our chaff as we go along.