Photographing glass or any kind of object that is semi or fully transparent can present some serious challenges.
In this really nice tutorial by Jay P. Morgan from The Slanted Lens, we are able to understand some of the complexity of a setup required to light a glass. The key principle is the same as if you need to light rain, for example. If you light water from the front, it is not that well defined. However, if you light it from the side or from the back in relation to the camera's point of view, it pops immediately. Glass is like water; it has a transparency property, and you need to light it from anywhere but the front. That's not all. You can back-light it and still not get a well-defined object. You need to control the size and shape of the area that illuminates the glass in order to get a well-defined outline.
In the video, Morgan shows three setups that can be executed with flash and continuous lights. You don't even need a powerful light beam to get the job done. On the contrary, you will probably have to power your lights way down, because the camera is looking straight at the illuminating sources.
You can apply the knowledge from the video not only to photographing glass, but also to any object that has some sort of a transparency like bottles or jewels.
Pretty basic stuff, but step one is to choose good glass.