People don't talk about shower curtain liners very often, but I find they make a big difference in your daily life as you climb in and out of the shower and try to keep the space clean. I've tried many different kinds, and my favorite for the past few years has been this one, which is lighter and softer than most others. It's also mildew resistant and not smelly as it doesn't off-gass (it's PVC free). I would recommend using it on the inside of a shower curtain as a number of other reviews say it's too light to use alone, but the lightness is one of it's chief benefits in my experience.
(Image credits: InterDesign)