Here's what we did last year for a 12v shower which we'll probably use again this year.
Take one large bucket. Fill with warm water (boiled kettles topped up with cold) place a 12v submersible pump in the bottom of the bucket (from a caravan or a bilge pump from a boat) and attach a shower head to it via a hose.
OK, now for the slightly more complex but much better version.
Take a hose from a direct source and fit a splitter so that you create two sources from the one. Run one directly to where you would like your shower (cold feed). Take the second source and connect it to a long coil of copper pipe bent round in a coil. Dig a hole large enough to house the coil and some charcoal. From the other end of the coiled copper, connect more hose and run to where the shower will be (hot feed) Place a flow regulator on to each of these sources, then put these into another splitter so that you have one single outlet with variable hot and cold water inlets.
Place the copper piping into the hole in the ground and set light to the charcoal. As the water passes through the copper it is heating by the charcoal fire and produces a steady stream of hot water.
As you couldn't get a constant water connection where we were camped, they actually ran thsi system from a huge trough fo cold water with a bilge pump so it was effectively a combination of the two.
Others we've seen on site are of course the shower bags heated by teh sun that you can get in any campning shop, and the large black containers placed on top of campers and lorries to warm in the sun and are then gravity fed.
There you have it, your own shower.