I use something from Zodi. Like this:
http://zodi.com/hot-showers/hot-tap-with-case. (A friend has the two-burner version which is better.) Mine is tip-prone and doesn't heat the water enough in one pass. Instead of following the directions, I use 2 5-gallon buckets, fill one, pump the water through the heater into the other, then reverse the hoses and pump the water back through the heater into the first bucket. By then it is fairly warm. I don't use the heater when I'm showering, I just draw the hot water from the bucket with the pump and shower from that. That avoids getting burned by the heater element especially if it tips over.
In addition to the heating setup, you'll probably want a shelter of some sort. One of the privy shelters will work. You can do without it but if it's raining or snowing that sucks. You'll also want some sort of mat to stand on to keep your feet out of the mud.
Another idea, which I haven't done, is to use a Coleman dish washing heater .. but be careful 'cause those dang things get HOT.