Soap scum is a soft, whitish build up & hard water stains tend to be more crusty, whitish build up. Unfortunately , both soap scum and hard water build up are both constant issues, regardless of what material your shower walls are made of. If your showers are ceramic or porcelain tile, there are many products available with which to clean them with and you can take your pick of those. If your shower is travertine, like the one in the picture, I recommend this product...
http://store.countertopspecialty.com/Stone-Care-Pro-BATH-SHOWER-Soap-Film-Hard-Water-Remover-1-QT-p170.html
it makes quick work cleaning stone services and does not etch the surface as some cleaners do.
Once the showers are cleaned, you can seal them-if you haven't already, with a product called STONE SEALER made by DuPont and is available at most home supply stores. It isn't going to prevent the problem ( there's no way to prevent it all together that I am aware of!) but it can make the stone and grout easier to clean. Very, very easy to apply- just follow the instructions on the bottle!

Thank you Floor Pro South! Great information! I will definitely try ALL those options. I was actually curious about any finishing services you offer, and may recommend. Any kind of "polish" you can do to the travertine? I've also heard the term "honing" thrown around. I post publicly these questions for the benefit of others that may be curious, as well. I will gladly contact you privately about any pricing info, if you recommend a service.

There is no polish available for travertine, that I am aware of, however, repeated sealings can give it a slight sheened appearance. If your showers are honed and filled travertine, it would be much more cost effective for you to buy the sealer and do it yourself, so it really isn't a service we offer. It mainly consists of spraying on the liquid, letting it sit and wiping it off, then repeating the process several times.
When travertine is cut into tiles, it is usually honed (ground to a flat, smooth appearance) and the holes are are filled in with an epoxy resin to give it a smooth surface, hence the description "honed and filled" you are familiar with. Tumbled travertine is usually NOT honed , and will have a more rustic appearance with holes and cracks visible because of that. Hope that helps!

Thanks again for great info!