Photographing verre églomisé is difficult for several reasons. Especially if the work contains mirror finishes. It's not unusual to see a nice image of the photographer reflected in the glass.
Strong lighting and objects behind the photographer can impart very dominant reflections that obscure the work. This is often the case with shop windows. The window may be under a verandah and relatively dark, which means that parked cars, and the reflected streetscape dominate the photo and make it difficult work out what you are looking at. You may have to try out several positions and angles to find one that provides reflections that are complimentary.
Sometimes a flash can be helpful in illuminating the work, but it is best if you can position yourself so that the flash itself doesn't reflect on the glass. Position yourself so that a window frame hides the reflection, or take the photograph at an angle instead of direct on.
Getting good shots in a studio is a little easier. The trick in getting a good photo is to have a neutral background.
I often hang a large grey cloth behind me as a backdrop to hide all reflections. If a white backdrop is used, it tends to wash out colours in the work. If the backdrop is a black, the colours a good, but the gilding appears dark and mirrors areas look black and give a false impression of how the work really looks.
Getting straight photos from directly in front of the work without showing the photographer or camera can be achieved fairly easily using software. I often take the photos at a slight angle, then just pull it straight in Photoshop. The other method is to have a little hole in your backdrop and shoot through the hole. If the reflection of the lens is positioned over a dark area of the work, it won't be noticed.
Some people have recommended polarizing filters. I haven't found them to be useful. They might stop incidental glare, but don't do anything to combat direct reflections from the glass.