Keeping high credit rating without debt
I've searched and found a few threads that mention what goes into the credit ratings. I'm fixing to pay off my student loan and last year paid off my car note and credit cards. All I will have left is the mortgage (which currently is good for tax deductions and too much to pay off soon). I currently enjoy the benefits of having a high credit score (translate - low insurance rates) and I'd like to stay there.
Will the following help?
Keep my credit cards open. All are at least 5yrs or older and then just use one and pay it off every month (I get points so I use it for purchases etc. to get the rewards as it doesn't have a yearly fee, but pay it off every month)?
Or is the above not sufficient? Do I need to carry a balance?
Anything else I can do to keep the score high (other than the obvious of paying on time)?