I've just finished laying one for a mate today.
Some manufacturers have layout generators on their websites but I've found they don't always follow guidelines for good practice. By this I mean that they have poor bonding patterns. I know that bonding isn't as important in paving as it is in brickwork but it usually means that poorly bonded paving is not as aesthetically pleasing as well bonded patterns are.
Some manufacturers have brochures that give suggested patterns. The one I finished today was by Stonemarket and used three different sizes laid in a repeat pattern like this one
http://www.stonemarket.co.uk/yorkstone_riven.a sp which looked absolutely fantastic when it was finished.
Once you decide on your stone sizes (the one in the link which I followed used stones 600 X 600, 600 X 300 and 300 X 300) you can workout your own layout using graph paper.