I would first try common remedies such as drinking milk (dairy, almond, soy, etc all work), or eating some kind of food that can stimulate melatonin production through seratonin. Here are some lists: Maximising Your Melatonin - Nutrition
Another option, is to take melatonin supplements along with Benadryl. Although, some people report feeling drowsy the following day, these things have been found effective to treat sleeplessness.
You might also want to consider some way to ease your mind. Often, insomnia is associated with anxieties. You may want to try meditation or seeking out a therapist or mental health professional.