A while back I came across a Life Hacker article on how to easily clean stove top grates by putting them in a zip-lock bag with a some ammonia.  The trick is not to soak them in ammonia, but to seal the bag and prop the grates up so that the ammonia fumes will do all the work overnight.  I gave it a try and it worked quite well, with just a little bit of elbow grease.

The only modification I made was that I used some disposable aluminum trays with plastic lids (something like this) that we had laying around and placed a quarter-inch board on the bottom to prop up the grates so they would be exposed to the fumes and not soaked.  I did one grate a night and never had to refill the tray with ammonia.

Before

After