You get the best rate by calling the hotel’s local number, not the 800 number. The 800 number usually links callers to an off-site, centralized call center that only has the standard room rates. You’re better to call the hotel directly and instead of asking for the reservations desk, ask to speak with the manager on duty, the general manager or the director of sales. They have the authority to negotiate room rates.
Rooms are more expensive in the morning. The best time of day to make a reservation is right after 6PM. This is when hotels wipe out all the ‘no-show’ reservations that were unsecured by a credit card. They then offer those rooms at bargain rates.
Everything is negotiable. Think parking is overpriced? If the lot looks half empty, offer less than the daily rate. Planning to make a lot of phone calls? Some hotels offer a per-day flat fee for local and long distance calling in the US (usually $9.95), but you must ask for it.