I sometimes need to take photos of new devices kept behind glass at trade shows. Currently I have to depend on my camera's in-camera flash. Often the picture is spoilt because of the flash's reflected glare. Any tips on overcoming or reducing this without resorting to a very high ISO?
Another tip you can try is to fit a flash diffuser on your flash, (a tip I was given is if you don't have a diffuser for the flash then use some of the semi opaque sellotape over the flash, as that will lower the potency of the flash), and fit a circular polarising filter to the front of your lens. Adjust the circular polariser till it removes the flash. This might take a few shots getting it right, but it does work.
There is another alternative: Use two polarizing filters, one on the lens and one on the flash, and rotate one of them by 90 degrees. The directly reflected light is polarized, and the flash reflection should be nearly cancelled out, where as your target produces a diffuse, non polarized reflection of the flash-light, which passes the filter on the lens. It's not perfect, but it should improve the results.
Take the photo at a high angle of incidence so that the reflection of the flash doesn't come back at the camera. That's your only option as far as using a flash is concerned. If you take a shot of the glass directly with the flash on the camera, it is always going to reflect back. You have to take a photo at an angle so that it doesn't reflect back. Ideally you would want to use an off camera flash, but that doesn't sound like an option in your case.
Alternately, you could setup a tripod and take a long exposure. If you take a longer exposure, it won't be necessary to use a higher ISO and you can avoid using a flash (which will probably get the best results), however you won't be able to hand hold the camera while doing long exposures without things getting very blurry.