You just have to get that paint/thinner and pressure mix right, and then just practise, practise, practise.
I posted a similar question a while back, and some of the advice that served me well was to count the time it takes to produce a mottle when pratising, and then apply that to your model. So if you take 2 seconds to produce a mottle, just spray for 2 seconds then stop. It sounds a bit stupid, but it prevents you overspraying and ending up with little spiders all over your model like I did at first.
Best of luck.
Karl