If you subscribe to Touring Plans you can have it search ADRs for you, you put in the restaurant and time you want and it will email and/or text you if they find one. It won't hold it for you so you'll have to then go on and make the reservation as soon as possible. Also, watch the cancelled ADR threads, sometimes people will post there when they have an ADR to cancel and you can try and coordinate a time where they will cancel and you can grab it. Other than that, just check here and there and see if any come up.