Revolving accounts like credit cards using them each month and then paying them off is good for your score, fixed accounts like mortgages, car loans, etc, when you pay them down/off, the amount of credit you have drops with the balance, and once it is paid off the account is closed, so it actually drops your score. If you don't use a credit card (and increasingly for me, if I don't use a card enough) the issuer will likely close the account or reduce your available credit, which will reduce your credit score...

So yeah, it is a pain in the rear to maintain a very high credit score.