Depending on what type of door you've got, most will have a means to adjust the the position of the metal plates that are attached to the door frame. Loosen the screws or allen bolts and move the metal plates inwards a couple of mm and by trial and error you should have a door that's easy to lock but draught free..
Rupe
December 2011
I can answer this question...
how to stop lots cold air getting in around pvc door?