How do you keep urine at body temperature?

I'd recommend keeping it in your body. ...Unless you have the need to preserve a warm sample of urine for some untold purpose. In that case, I'd recommend keeping it on the stove in a small pot which you will never use again for anything else. If you have a long way to transport this sample for whatever purpose, I'd advise heating it well over normal body temperature to allow for cooling on the trip.