Ever take pictures of something behind glass? Maybe at a museum, an aquarium or even on an airplane. You will almost certainly get a reflection of you or something else in your image - and if you use a flash it will be even worse.
Simple solution is to put your lens right against the glass barrier itself. This will shield out the stray light/reflection and give yourself the best possible chance for a decent shot. If you have a lends hood, you will most likely have to take that off too depending on its shape. Be careful and take your time and your lens will be perfectly fine.
This tip is pretty obvious but wanted to share in case you have never tried it.
Here is one I took at Dulles Int’l Airport from the comfort of the United lounge. You would never know there was 2 inches or more of glass between me and this scene.