How can I raise my body temperature?

There are a few different ways you can raise your body temperature.  I guess it just depends on what your purpose is in raising your temperature.  
You can take a hot bath or shower, get into a sauna or hot tub, apply a heat pad to a desired area (though that will only penetrate a couple centimeters deep), or you can simply rub your hands together to produce the heat caused by the friction.  Still, the very best way to raise your body temperature is to go for a light jog - just get your heart pumping.  Jogging/running is one of the easiest ways to get your heart pumping in a short amount of time and works throughout your whole body (as opposed to the heat pad). 

A thorough and effective warm up will allow you to raise your core body temperature in preparation for physical activity. For a thorough warm up the NASM recommends starting with self myofascial release of over active or tight muscles, followed by static stretching to lengthen excessively tight muscles. This is followed by active stretches for these muscles, and finally a dynamic warm up of the whole body to help prepare the body, the muscles, and the nervous system for more advanced and complicated movements.