Apple - How can I keep my iPhone / iPods cooler in a hot vehicle?

Place the electronics that contain a lithium based battery in the shade and keep windows open a crack. If possible, place the vehicle in the shade or use light or reflective colored windshield shades to reduce the heat that stays in the car.

Usually the trunk (or a well insulated bag) will lag the passenger compartment many tens of degrees and warm up several hours slower than something in the main cabin.

Coffee cups (with lid, of course)

The most flexible and cheap solution are coffee cups. These cups are usually designed to have insulating walls.

Insulation is the most effective way to prevent heat building up. You might want to prefer white cups, but it should not make much of a difference.

Insulation, not only can it keep stuff warm for longer, but it can also stop stuff heating up in the first place. So anything that provides insulative properties, styrofoam cups etc can be used. Even a cardboard box.

The first rule of thumb is to keep the thing out of direct sunlight, and even if it's on a pocket or compartment, choose one that is also not directly shone upon.

Another thing would be to turn if off while unattended in the car. If you have left it in the car, you can't be actively using it, so turn that sucker off and save some battery while you are at it.