Once the interior is comfortably cool, there's a limit to how many degrees temperature differential a refrigeration system (like your air conditioning) can maintain between the hot exterior and the cooled interior. My experience with older cars puts this at around 30 degrees F for a well functioning system. When the car is traveling at highway speed, you should have adequate cooling airflow, so as not to overheat the engine, so the only accessible factor is how fast your car accepts heat from outside. This depends mainly on two factors: how much solar heating gets in, and how much heat conducts through the body work from the outside air.
Applying a reflective window film can help reduce solar heat uptake, but may be restricted by law (North Carolina, for instance, requires that after market window tints must transmit some minimum amount of light, and tests this for an extra fee at annual safety inspections). A low-absorption roof finish of some kind (white paint absorbs less heat than silver, and causes less glare for nearby high vehicles) can greatly reduce solar heating through the roof.
Another improvement would be to insulate the roof by applying foam between the headliner and roof. Similar insulation could be applied inside the door panels. Combine this with a mirror-finished roof and windows as reflective as your local law allows, and you might increase your maximum differential cooling by ten degrees or more -- which could be very significant on a long drive in high ambient temperatures (like summer in the South or Southwest of the United States).
If you don't need the extra cooling for comfort, you can save fuel by running the A/C at "Max" or "Recirculate" setting, and turning the compressor off and one to regulate cabin temperature -- the power draw of the compressor is the same at "Max" as at any other temperature setting, so turning it off for a while now and then will save fuel compared to running it constantly and adding engine heat back into the chilled air for comfort.