If your sure that your heater is blowing normally, then all you can do is the following:
1. Remember that the easiest way to demist the windscreen is simply to open the car's windows ;
2. Ensure that the screen is REALLY clean. The water droplets that form the mist on your screen form more quickly (and adhere better) if there's a thin film of grease on the screen. Call in at any supermarket and buy a bottle of 'value' vinegar. Clean your screen, both inside and out, with it. (Your car will smell like a chip shop for a day or two but it's worth it in the interests of safety!) ;
3. Treat your windscreen with an anti-mist solution. See the reviews here:
http://www.practicalm...om/product/apr07.html
Chris