What are some natural substances that promote fertility?

Stay healthy. A healthy diet, limiting caffeine and alcohol, avoid tobacco, moderate exercise, and staying healthy promote natural fertility. Herbs, otc drugs, diets, vitamins, supplements, and tricks - natural or otherwise - to 'make you fertile' don't work and can be dangerous.
Antioxidants. Antioxidants reduce the reactive oxygen species in semen. L-carnitine 100mg daily and coq10 200 mg daily both improve sperm motility. Avoiding heat sources including laptops on the lap, limiting cell phone use to less than an hour daily are also good ways to promote sperm quality.