After many near-misses with the supplied wrist strap I've decided I have to use a neck strap with my S90. I want something light, unobtrusive and comfortable. Currently I'm using thin elastic cord but it cuts into my shoulder after a while. Can you recommend anything else? Form over function - looks are irrelevant to me.
Canon actually makes two straps that are really designed for this size camera and purpose. They also have ones that are designed more for DSLR cameras, but provide more substantial size and thus may be more comfortable to you.
The Canon Leather Neck Strap for the PowerShot S95 is actually designed for the S90 series cameras. This will work perfectly for that size camera, and what is great is that it is made of leather so it will be very comfortable.
Another option is the Canon Metal Neck Strap which is spec'd out as working with any ELPH series cameras, but the S90 is nearly identical in size, and I also found reviewers who said that it worked great for them on a S90/S95.
Finally, you could go all out and choose a DSLR strap such as this Canon Professional Strap. Sure, its bigger, but if you want comfort this may be the best option.
Personally, I actually would use the leather strap that is designed for this series of cameras, but it is personal preference obviously.