Here's a tip on how to install a pre-hung door. First, lift the door into place, then be sure it's plumb. Always use your level on the hinge side. Nail through the casing and remove the factory clips, so you can test the swing of the door. Then shim the door with scrap shingles along the hinge side and secure the jamb. Now you're ready to put your finish moldings in place.
For more on doors, consider:
10 New Uses for Old Doors
How To: Install a New Door
Know Your Door Styles: 10 Popular Designs