How to Create a Rainwater Collection System for Your Greenhouse

I want to stop having to go all the way over to the outside tap or drag the hose into the greenhouse when I am wanting water the plants. So, I am going to install a rain barrel/water butt and then rig up a system so that I can get the water piped into the greenhouse itself. This will, being "The Optimistic Greenhouse" involve something to do with pallets!