This idea said to put them in a zip-loc bag with some ammonia. You don't need much, the fumes work on the grime.
I used 2 gallon zip-loc bags and put 2 grates per bag with about 1/4 cup ammonia. Then you seal it up tight and place it somewhere overnight. Here mine are, sitting in the tub for the night. (I wish I had thought to take a picture of them before I started. You'll have to take my word that they looked pretty bad.
After sitting like this overnight, you were supposed to be able to just rinse them off and they would be clean.
Mine needed some scrubbing after being rinsed. However, when I've scrubbed before, I never really got anywhere. This time, the crud came off fairly easily.
Here's the after shot - all nice and clean! I will definitely use this pin again. Next time I think I will just put one grate for bag though. I think they'll clean better.
I would recommend trying this one out!