Not always the easiest to accomplish, but worth the effort!
Cosmo provides these “recommendations” that cover everything but actual intercourse: http://www.cosmopolitan.com/sex-love/advice/a3628/shower-sex/
My favorite comments come from this article (http://www.lovepanky.com/sensual-tease/seduction/how-to-have-sex-in-the-shower) because it includes “safety” recommendations like holding onto something.This one is fun as well, it has a grown man crying in one of the photos: http://www.bustle.com/articles/47949-sex-in-the-shower-the-ultimate-guide-to-getting-your-super-soaked-freak-on
In the end the “experts” seem to agree, it will depend on the size and shape of your shower; but here are 10 positions to try: http://www.yourtango.com/2015275297/10-sex-positions-you-need-try-shower-your-man
Remember, the shower part may help your sex life as well: