if you want to go really natural then lavender, its very relaxing. To be honest I think the calming scent which isnt too stimulating helps you focus on your breathing, have a bath and put a few drops of lavender oil in and then either add a few drops to your pillow or a tissue, alternatively you can buy little sleep pillows stuffed with dried lavender.
I used something once called 'Quiet Night'. I took the recommended dose but I couldn't wake up properly the next day. It was only about 3pm that I felt as if I had woken up.
I swear by herbal tea, Apple & Camomile by Twinings. A cup or two of that and you'll be gone. 20 bags for about 85p
If I really really want to have a good nights sleep I'll have the above tea and a couple of Nytol, Kalms remedies.