Heat the griddle up until it is smoking hot - it starts to burn off a bunch of junk. Then give it a squirt of water to get a nice steam going - steam cleans. Then lightly scrape it with your choice of tool. Give it a minute and hit it with water one last time. Reduce the heat to low, wait 10 minutes and put 2 pounds of bacon on it and render down. Drain bacon fat into grease trap and call it a day. Make BLTs when done.
When all else fails rub some bacon on it: