When installing a new door, use shim shingles as extra support between the jack studs and the door jamb. Use two cedar shingles, one vertically and one horizontally, adjusting until they fit snugly. Use three sets of shims on each side-one on top, one on the bottom, and one in the middle. Use three screws to fasten the door jambs to the frame.
For more on doors, consider:
Hanging Double Doors
Know Your Door Styles: 10 Popular Designs
Shimming, Centering, and Hanging a Door (VIDEO)