Actually, if you read my contract, it states that there is a minimum monthly fee. Like nwilson I only usually have to do updates for a 2-weeks to a month after it's released, but since I manage all hosting, technical emails to the site, and they always have me on-hand I charge a certain monthly fee.
Not only does it help to establish a long lasting relationship with a customer (seriously! Whenever I bill them each month, I also write them a letter about how there website's doing, how to improve it, etc.), but it also creates a more regular income. I usually charge 20% profit on set-up, and then, updates aside, monthly fees are almost pure profit (though I do use those to cover our month-to-month costs like phone, fax, hosting, etc.).