Replace the Filter
Many times an air conditioner freezes up because the air isn't being exchanged efficiently through the system. Something is restricting the air flow, and that is usually dirt in the air filter.
AC filters are similar to those in furnaces, in that they are rectangular, easily removable and replaceable, and get clogged with dust and dirt quite frequently. Replace your air conditioner filter once a month during the summer to help keep the air flowing through your unit.
Open the Vents
A surprising number of times, an air conditioning unit ices up simply because the vents in the rooms aren't open, again restricting airflow. Open the heating and cooling vents in at least most of the rooms in your house.
This helps to keep the air circulating freely, keeping your unit at an even temperature.
Clean the Outside Unit
Central air conditioning units generally have a large rectangular box set up outside your home. This is the main AC unit.
Often, foreign object get blown against the box, clogging the intake filters and preventing the air from circulating. Check for items like cottonwood fluff, dead leaves, blown newspapers and other solid substances that may have ended up against the unit.
Sweep it off with a broom weekly to help prevent clogging.
Call a Technician
If your system is clean and the air is flowing correctly, you may have a leak in your coolant coil or a clogged coil. Call an HVAC technician to check your unit.
He can clean out or replace coils and replace the refrigerant if necessary.