To oil a grill rack: Oil a folded paper towel, hold it with tongs and rub it over the rack. (Do not use cooking spray on a hot grill.) To grill with wooden skewers: Wrap the exposed parts with foil to keep them from burning. (Contrary to conventional wisdom, soaking skewers in water doesn't protect them.)
Combine cilantro, parsley, garlic, paprika, cumin, salt and pepper in a food processor; process until the herbs are finely chopped. Add yogurt and lemon juice; process until smooth. Scrape into a medium bowl, add lamb and toss to coat. Cover with plastic wrap and marinate in the refrigerator for 20 minutes.