kspencer, was it just the a/c that was replaced, or did your contractor replace the furnace (air handler) as well?
Did the contractor do a load calculation before the equipment was quoted?
If the load was properly calculated, all you need to do is have an HVAC professional perform semi-annual preventative maintenance which will include checking airflow, refrigerant pressures, condenser coil cleaning (and evap coil if necessary), among 20 - 30 other items (depending on contractor). All you really need to do is change the filters regularly and set the thermostat to an appropriate setting (htg- 66 to 70 deg F., cooling 72-78 deg F.).