keep the soil moist, never soaked and never dry during seedling stage. also helps if you water them gently as their roots aren't well developed yet. it might help too if you take an aluminum can, cut both ends off (serrated steak knife works well), and jam one end into the ground a bit, centered over the plant. it'll keep away cutworms and other crawling insects that love tender sprouts and can be left in place or removed later.