Window won't stay open?

My Double glazed windows open outwards and are of mock sash style, the problem is that when open they get blown shut really easily and the downstairs one doesn't stay open more than 5-10 cm before closing. Does anyone know how I can fix this so they stay as wide as I open them? There is no resistance anywhere to stop them closing, so I have to wedge something in the way to keep them open! Any tips greatly appreciated!!