I went last year for the first time ever to a festival and was also worried about the washing and showering problems. I need not have worried as this is what I did.....
Got one of those solar showers and filled it half with cold tap water and boiled the kettle a few times for the other half. I then sat in a small round kids paddeling pool just inside my tent (could do it outside it you wore a bikkni!) then got someone to hold the shower bag above me to let the water come down on me while I had a quick, but sufficiant wash with shower gel! Can also wash hair like this too.
So easy and felt good as new every day!!