Get upgraded for free. Price isn't the only measure of a great deal! To improve your chances of getting an upgrade, wait until about 6pm to check in. By then, the front desk should know which of the nicer rooms are still vacant and you'll have a better chance of scoring one of them—minus the extra cost.
Make a business decision. If your heart's set on a weekend stay, head to the business district for real value. Many hotels in corporate, financial and conference hot spots are busiest midweek and offer better prices on the weekends. (They tend to be close to public transportation, too, so you can save on cabs, cars and gas.)