Like this post?
water's a crappy lubricant for one, i wouldn't reccomend it at all myself, most home showers barely even fit me alone (i'm 6'4") so short of remodeling, i have 2 suggestions...
1) you can buy accessories (straps, handles) to make it easier/more comfortable, be sure you have a non-slip mat.
2) cheeper solution, cowgirl position. (you on your back, GF rides you) anything horizontal and you can get poked by knobs and nozzles, never fun... vertical you CAN hit the showehead, but it's really unlikely.