Changing your filter once a month is best thing you can do for your Air Conditioning system. All the air in your home goes through this machine and the filter protects your machine from air-borne contaminates and YOU! If you have pets in your home you should consider changing your filter more often.
Keeping the outdoor condenser free of leaves, dirt and and other obstructions is also very important. Air needs to pass through your condenser also. The more air movement around the A/C condenser the better your machine will cool.
Yearly maintenance is the key to catching small problems before they become BIG problems.