The best time to clean your BBQ grill is after you use it. Once you are done cooking, leave the grill on for another ten minutes or so (this is especially easy if you are using charcoal) so that any stuff left on the grill can get nicely burnt on. Then take a wire grill cleaning brush and use it on the grill. Everything will come off very easily. If you let the grill cool down, it'll be a lot harder to clean.