If you lock in a mortgage rate, make sure the expiration date leaves enough time to complete the real estate closing. The flood of home purchases and refinancings has created major processing delays, so if there is any question about the processing time, or if you seem to be cutting it close, you may want to pay a little more for a longer lock-in period rather than risk losing your rate.
Change the filters regularly. Dirty filters restrict air flow, reducing efficiency and worse case, can cause the evaporator to ice up. Disposable fiberglass filters should replaced. Electrostatic or electronic filters need to be washed regularly.