Since our daughters were quite young (maybe 2 or 3 - I'm not really sure now), I helped them to get to sleep by putting them (each) in a bubble. This involved talking through a basic relaxation visualisation, where their auras were flooded with colour, while running my hands over the aura (like sealing the bubble). Initially, I'd suggest what colour they should see themselves bathed in, but eventually they could choose based on their mood (with minimal direction from me, such as steering her away from black or red - not great bedtime colours).
As well as the bubble, when they were a little older I also taught them some other visualisations or meditations to help them drift off. We also gave them a CD of children's guided meditations (from Toni Carmine Salerno), which they really enjoy. I've found that from doing these simple sleep exercises, they've become quite proficient at using their breath to calm themselves down. My hope is that this will prove to have set them up well in life