Using your car's air conditioner costs a lot these days in gas money. It's far better to just use a few smart ideas to keep your car's interior cool in the first place.
Try to conserve gas by parking your car properly, allowing for cross-breezes, and reflecting the sun's rays away from the inside of your cabin. If you can keep the interior as cool as possible, you can go longer without running the air conditioning, or at least decrease the heat load the AC has to move out to cool you down.
1. Park in the shade. Not only is it a good thing to park directly in the shade, but you can also look at the time and think ahead to see how long said shade will be on your car. If possible, at least try to get the dash area in the shade.
2. Crack the windows. If you can get some kind of cross breeze through the car, that's very helpful. The tiniest crack will do, to keep your car secure.
3. Use a sunshade. There are tons of sunshades that pop open, expand or simply drape across your dash area. It's crucial to keep the dash area cool, since the glass and plastic areas so close together act like a solar oven. A large enough sunshade will cover the entire front seat to help keep things cool. I have found good thick metallic bubble plastic sunshades at dollar stores. Anything you use is better than nothing, however.
4. Reflect out the solar rays. If you can get a sunshade with a shiny finish facing outside, you will do even better. Add actual aluminum foil and bubble wrap to the outside of a cardboard shade if you must. Think about being reflective to the outside and insulative on the inside.
5. Create a sunshade or curtain that covers other windows, or use a mirror film coating to bounce solar rays away from the rest of your car interior. Be creative with fabrics, throw a towel over the cracked windows, sew yourself something light colored but still light-blocking to hang up, or adapt a dash shade to fit other areas of your car. Shading a hatchback area is a good idea.
When you get in the car to drive, open the windows and outside vents to let any cool breezes sweep the hot internal air away. Wait as long as you can before turning on the AC - you might find that for that trip, you won't even need it. Being mindful of whether you actually need the AC will save you lot of money on gasoline.