Having trouble photographing your flat-crystal watch (such as Seiko, Tag Heuer and Rolex models)?
No problem! get yourself a circular polarizer filter and place it over your lens of choice. Slowly rotate it while watching through the viewfinder to make the crystal disappear!
You will loose 1 to 2 stops of light (depending on the filter) but they are Neutral density, so there is no color correction needed.
This trick also works on curved crystals and 'cyclops' magnifiers, but to a limited extent.
Also useful when you want to enhance the reflection for other effects.
NOTE: this trick is for use with SLR cameras only. It will work with non-TTL viewing, but it requires more trial and error. Not a big deal in this digital age, but just thought I'd mention it.