Polarizing filter.

Maybe if you bracketed a few shots and put them together as an HDR?

Polarizing filters do not help much at normal incidence- that is when
the glass is at a right angle to the camera/lens axis.  So you need to
use staged lighting to eliminate the reflections.  This is admittedly
hard to do- one of the harder lighting tasks. It is almost easier
sometimes to remove the object from the glass.

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Reflections are a pain.  Generally the problem is that
for each and every light source, there are an entire set
of reflections from any particular glossy surface.
Hence each window will have a reflection, each ceiling
light will too, and so will any other source of light.
If you move the camera to get rid on one reflection, you
are sure to then find another (or half a dozen others).

The trick is to reduce it to just one single point
source of light.  For that, a relatively dark room is
idea, so you have a good chance!

Use a single flash, located relatively far away from the
posters, and do not use any kind of reflector or diffuser
on it.  That will make it appear as a single spot on any
reflective surface and may allow you to position the
flash and the camera such that there are no reflections
in the image itself (it might only allow you to reduce
their effect though).

You also want the flash to be much brighter than any
other illumination, so setting it for manual at full
power and then using manual exposure on the camera will
help too.  And if possible, cover the window and turn
off other lights!

You'll almost certainly have to move around a bit,
taking multiple shots to view the results looking for
positions that result in the least amount of visible
reflection.   A modeling light might help a bit, but
it won't be bright enough to see everything, so it's
just a case of shoot and view, shoot and view until you
get it right.

Polarizing filters will not remove specular highlights or reflections
like this. Ideally, you would use screens to block any unwanted light
and you might even use a view camera to offset the camera enough to one
side to eliminate reflections. A simple perspective control lens
*might* be enough, but a view camera would be better. Offsetting the
camera to one side will also keep the reflection of the photographer
out of the picture.

Or you could Photoshop it. Take the image from two different angles, so
the reflections appear in different places on the image. Then simply
erase the unwanted reflections on each layer. See "The WOW! Factor" in
the April/May issue of Photoshop User. Since you are moving the camera
and not the reflections, you will have to use Transform to get the
images to align closely enough that Photoshop can merge them.

One thing to try is shooting off axis then straightening it out in
photoshop. You will lose some resolution of course.

Use two flashes at 45 angle to the poster on each side, use fastest sync
speed and a small aperture to reduce reflections from ambient light to
negligible brightness. Some lense become unsharp at smallest aperture so
this is a compromise.

If you have only one flash take one photo with the flash on each side then
blend them together; use manual exposure and manual white balance so the two
photos will match exactly.

Hopefully there are no dust or scratches on the glass.

Use google to check out "copystands". Same principle.

Mmm, you are a bit screwed really.  I would try to talk to someone in charge
and try to come up with a deal with them so that they can be removed to
photograph them.

Usually if the light sources are off axis (30 deg for more, for
instance) you should have no problem.

If you can mask the light sources so they don't shine directly, that
will do it.

I'm assuming that you're shooting from a good distance back?  I'd
suggest at least 85mm, 135 - 200 is ideal if you have the space to do it.

Light hitting at 45 deg or more off the lens axis should not be a
problem at all.

A polarizer can help, esp. with a longer lens (85mm or so).

If there is a light source behind you, then mask it off.

Here's some good info, you'll need to be able to open a PDF file:
http://www.aucklandartgallery.govt.nz/downloads/standardtechniques.pdf