My friend found cling film with medical tape holding it down worked well. The tape is water proof enough to last a shower and stops the cling wrap moving too much. Also holding the shower head so you get to control the water spray and using a flannel will help.
I cut the bottom off a garden rubble sack and put the handles over my shoulders (like a bin bag dress), I stood with my back facing the shower which meant the plastic at the back stuck to me making the front really tight so that when I turned around my dressing stayed dry. I need more surgery so will use this trick again in a couple of week's time. Hope it works for you - weird as it sounds!
