For mysterious, quantum-mechanical reasons, the screws that hold handles and doorknobs always eventually work loose. A few drops of thread-locking compound will permanently fix the problem, yet still allow you to remove the screw with ordinary tools if you need to later. A heavier-duty variety is also available for large bolts and machinery.
Mark, Don't Measure - Holding trim in place and marking it is always more accurate than measuring, often faster and iteliminates mistakes. This is good advice for other types of carpentry work too, like siding, laying shingles and sometimes even framing.