Preload doesn't affect suspension stiffness, it simply adjusts the amount the suspension sags when the rider sits on the bike. You need to adjust the preload so that around 25% of the total suspension movement is taken up when the rider sits on the bike. You can also use preload to alter the ride height of the bike.
See here for some more info..
http://www.promecha.com.au/sag_and_preload.htm