I bought the 3.0" GGS DC LCD Screen Protector, which had universally good reviews. It's a thin, rigid multi-layer piece of glass that looks to me like the same sort of material in front of DSLR LCDs (Note that with most DSLRs, you don't need one of these protectors, since the one that comes with the camera is not difficult to replace).
The 3.0" size is a perfect fit for the S90. Since it's rigid, installation is very simple:
- Clean the LCD and the area around it
- Remove the adhesive backing from the protector
- With the camera on, line up the protector with the frame of the LCD
Since the protector sits on the frame and not the LCD, it adds a slight bit of depth. I found this to be neither a positive or a negative; it makes the view more like my DSLR's. The clarity is perfect, far better than those flimsy plastic LCD barriers.
Best part: The GGS protector costs only $6 shipped, on eBay.