Another reason systems may appear not to be producing enough cold air is because of duct leakage. Duct leakage can sap 20 to 40% of the energy out of even a well-operating air conditioner, if the ducts pass outside the cooled space (this includes attics, crawlspaces and garages). Ducts outside need to be well insulated.
Sealing leaky ducts may be the biggest single thing you can do to improve efficiency, but a lot of the issues mentioned about will help as well: replace dirty filters, keep the right charge and airflow, clean the coils.
Another thing to do is to make sure the outdoor (condenser) unit is not so hidden from sight that its air flow is blocked or that leaves or other matter are not clogging it.