It really depends what the haze is. If it's some deposit on the surface of the glass, soaking in a solution of biological washing powder may help, followed by a good scrub with a stiff brush. However, I think it's much more likely that the haze is from water damage - if that's the case, then, as Taylog said, the only way to remove it is to mechanically polish off the damaged surface layer. There are a few threads on how to do this so it should be worth searching the board. Personally I've had great results with a cerium oxide slurry on a polishing bob driven by a Clarke rotary tool (similar to a Dremel but cheaper).