You could try a polarising filter to cut down on the reflections, but the best way I've found is using the lens hood as close to the window as possible and using a black clothover it to cut out as much reflecting light as possible.
There is always a tent like gadget (can't think of the name or better way to describe it) that attaches to your lens and suction cups to the window so you get a clear view, or similarly get an extended rubber hood that you can hold against the window to cut the reflections. Not a good idea to wind the window down, unless you're feeling very brave that is.