(1) Air filter(s) MUST be clean. They should be located near the return air duct adjacent to the air handler or in a return air grill(s). Check your air filter every 30 to 90 days to make sure it is clean. Depending on the house, i.e. if you have animals you may need to clean or replace the air filter(s) more often. (Go clean them now!)
(2) Set the fan switch on the thermostat to FAN ON, not AUTO. This will run the indoor fan nonstop. The outside A/C unit will still cycle with a call for cooling from the thermostat. The constant air moving will keep you cooler. You can probably keep the thermostat a degree or two higher than normal and still feel comfortable. You will also maintain a more even temperature between upstairs and downstairs. This will SAVE you MONEY because the outdoor condenser will not come on as much!
(3) Make sure that you wash the outside condenser coil once a year. If it's dirty the A/C will run hot and inefficient.
(4) "Warm Rooms" on the lower levels of the house where it is cooler cut back / partially cut off some vent registers (Diffuser) but do not close them off all of the way, doing so could interfere with the static air pressure (air balancing of the system.) Make sure that all the ones on the upper floors where it is warmer are open all the way! Also, see paragraphs #2.
(5) "Doors" if you close the door to a room make sure that there is about a 3/4" gap between the bottom of the door and the floor. You may have had carpet put down on the floor and now there is no gap. This is necessary if you have a central return air duct in the hallway. The return air ducts need to pull the warm air from the room.
(6) Never leave the house and turn OFF the A/C. then come back home and turn it on and expect it to cool the house anytime soon. Doing this will not allow the unit to cool down the house for several hours. You can set the temperature up five to ten degrees but NOT OFF. This is because of Latent heat buildup in the walls and furniture in the house and will make the A/C work harder to remove the heat, this takes a long time.
(7) Never turn the A/C off than back on in less than five minutes, this will short-cycle the compressor and can trip breakers, blow fuses, or cause permanent damage the compressor. You should have a time-delay install on the A/C to prevent this during power outages! Most digital programmable thermostats have a time-delay of 3-5 minutes built-in. Having a start capacitor and relay is a good idea. This will increase the life expectancy of the compressor by starting faster thus keeping motor temperature down, using less electric to start.
(8) Keep blinds closed, curtains drawn, window shades down. A working attic fan would be a good idea. Plenty of insulation in the ceiling & walls. Air tight storm windows. Keep outside doors and openings close, etc.
(9) Never cover the A/C (Outside unit) with plastic or an air tight cover; this will cause it to rust.
(10) Do NOT let animals (Cats) (Dogs) etc. Urinate on the outside coil. This will cause it to corrode (rust) and then leak refrigerant.
(11) Keep grass, leaves & weeds from blocking air flow on the outdoor condenser.
(12) Do NOT build a deck close to the top of the outside A/C or anything else that could cause the warm discharge air to re-circulate back to the unit.