Prevent polytunnel from blowing away?

I would like to invest in a polytunnel but am a bit worried that it might blow away (as happened to our trampoline along with its protective net, a few years ago). How can I minimise the chance of the polytunnel blowing away? What is the recommended orientation of a polytunnel, North-South, East-West ?I believe that our prevailing winds are from the South West. Thanks in advance for any advice you can pass on.