What are natural remedies to prevent insomnia?

Many. While there are many over-the-counter remedies that are used for treatment of insomnia, the best prevention is to have a good sleep schedule (also called sleep hygiene). Go to bed and get up as close to the same time as possible, do not drink caffeinated products or alcohol at night if possible and sleep in a cool, comfortable environment free from light and sounds. A good bed doesn't hurt either.
Many options. Favorites include 5-hydroxytryptophan, theanine, magnesium, & herbs like valerian, passionflower, skullcap & hops. Ashwagandha & tulsi tea can also help, as can homeopathic remedies. Avoid caffeine later than noon. Exercise in the morning and meditation in the evening can help. Minimize stress & avoid excess stimulation before bedtime. Consider a hot bath with lavender oil & relaxation cd's.