I live in the tropics, and the temperature here is over 30C most days. For nearly half the year, we have heavy rain almost every day, and the rest of the year, we have almost zero rain.
I have a VooDoo Omega, which I imagine is very efficient at absorbing heat.
My No.1 strategy is to park in the shade: a little effort, but basically, a no-cost option.
No.2, I put a reflective screen up inside the windscreen - a few dollars at your favourite auto-shop.
No.3, if I don't expect rain, I leave the windows down 2cm - at least two windows, to get ventilation across the car (inlet and outlet). This makes a significant difference even on short stops, and costs nothing.
When I get back to my car, and it is uncomfortably hot, I turn the air-con off, wind the windows down, and get some serious ventilation happening while I'm driving. I believe that using the air-con to compete against 1.7 tonnes of absorbed heat is futile: it costs fuel, but doesn't acheive much. After driving for a couple of of km, even in town, the car has cooled sufficiently for me to put the windows up, and turn on the air-con.
Before my VE, I had a VZ, and one way that I think Holden went backwards from the VZ to the VE was the door-locks. The VZ had two-stage, but the VE only has single-stage. When I left the windows down on the VZ, I always double-locked it, but I can't do that on the VE. So, I am left with the tiny suspicion that some clever, dextrous, theiving bastard will open my door through the 2cm gap that I leave: no such fear in the VZ. Mind you, when I discovered that the VE doesn't double-lock, I had good look at the set up, and it would require an exceptionally dextrous and/or clever theiving bastard to manipulate my door locks.
On long trips, I drive with my windows up, and air-con on - that usually works until I get well into NSW. I sometimes wonder whether I might use less fuel with the windows down and the air-con off, but I expect that that would adversely affect the aerodynamic efficiency (that Holden paid millions to design and test), so I'm better off with the air-con.