The best places to affect internal temperatures from the outside are your head, your hands, your feet, and your crotch. Lots of bloodflow through those places, and a relatively good amount of surface area to transfer heat. (Taking it the other way, in winter, if you want to stay warm, you cover your head, hands, and feet.) The wrists and neck don't provide a lot of surface area, but at least you have quite a bit of blood flowing through those areas. The small of the back is a pretty poor area to use to affect your internal temperature.

Of course, it's also a good idea to dump cold water inside of you (as long as it isn't too cold too fast).

There was a show about DARPA on TV the other day where they showed a device that circulates hot or cold water around the subject's hands, cold water to cool the subject and hot water to heat the subject up. The device started to change the subject's core body temp within 30 seconds of the subject putting his hands into the device.