Pull the grate from your grill and place it in a bed of stone or large gravel.
Massage it into the stone so it rests with about an inch of stone on top.
Commence rubbing of stone across the grate.
After a quick rinse with soap and water, your grills are ready for use again.
Check out the guide at MyFixItUpLife for more details and pictures of the process.
How To Clean Grill Grates| MyFixItUpLife