Originally Posted by cepheid
MileageManager will automatically notify you if your miles are about to expire. Also, I believe Randy said once that an upcoming feature actually will do what the OP wants: deposit a few miles to keep the account active. I'm not positive about this, though, so perhaps Randy can chime in.
Not sure how the "mileage deposit" could possibly help you with programs like Starwood and Amtrak which require actual stays (or an SPG Amex) or train trips to extend the life of your account. I'm sure there are others that have similar requirements.