I've tried scrubbing them with cleanser and steel wool and they just never seem to get clean.
Last night I decided to put one in a gallon size ziplock bag with pure amonia, ( I keep amonia in a spray bottle ), so I just sprayed into the bag so the grate was covered, zipped it up and let set overnight.
This morning I just used one of those heavy duty green scrubber pads, and with very little elbow grease, all the burned on stuff came right off. I now have another one soaking in the bag to clean this evening.
I've heard you can set a dish of amonia in your oven overnight to make cleaning easier, but it never worked that well for me. I guess the bag keeps the moisture in helping to dissolve the burned on grease.
Before picture:
