** You need to make the vent pipe air tight. Use a good quality duct tape to seal the seams. Then, as Andy says, insulate the pipe to prevent any consensation inside the pipe. **
I agree but the OP should also make sure the vent length is not too long to the dryer model you have to handle (see the following link) and also that it is not partially or totally plugged, plus that the vent cover is working properly.