I use a steam cleaner. Combined with a scrubbing pad, it works pretty well. (A non-abrasive scrubber like . Something harsher might scratch the glass.)
I tried a strong solution of washing soda only yesterday, whilst it got rid of most of it, the really burnt on splashes resisted. Maybe I needed an even stronger solution...Mind you the green scratchy cloth did get rid of a lot of it, but I got bored...I'll try breadsoda tomorrow.