First thing you must change your furnace filters and also keep grass and bushes from growing around  the A/C unit .You can also check to see if you A/C is clean if the coil is dirty on you central air  [ A/C unit ]  you can wash it  with your garden hose. PLEASE!!! shut  the A/C unit off  and either pull your fuses or shut off your breaker at fuse box before doing anything with the central air to prevent getting shocked .Now once you did that take your garden hose and proceed  to wash the central air,  the outside coil only  using care not to bend or damage the fins. Wash  the A/C unit at a 60* angle starting from the top of the fins and washing down  going around the A/C unit till the whole coil is clean .Wait 15 minutes and return fuses and turn breaker on then turn A/C unit on setting thermostat to desired temp , and  then enjoy the fruits of your labor.