Check pressures at least monthly and before any long trip. Always check when the tires are cold - driving even a couple of miles warms the air and inflates the reading.
Use the figure on your vehicle's placard, usually inside the driver's door frame or in the fuel filler flap. Do not use the number moulded into the tire sidewall - that is the maximum the tire can take, not the pressure your car wants.
Under-inflation makes the shoulders wear faster, increases fuel consumption and builds heat, which is the main cause of sudden failure at speed. Over-inflation wears the centre and makes the ride harsh with a smaller contact patch.
Do not forget the spare, and remember many cars specify higher pressures when fully loaded. Air checks are free at any Tire Zone branch.