Replace an Interior Door Handle: $15 & Up
When cleaning the mechanism isn't enough to keep a doorknob from sticking, it's time to replace the knob. Remove the screws holding the cover plates to the door, then pull the doorknob apart. Outside the door, insert the stem of the new doorknob into the latch case -- the mechanism inside the hole in the door. Inside the door, fit the interior knob over the spindle and align the stems with the screw holes. Install a screw into the hole closest to the door edge. Install the other screw, and test the doorknob. If the latch sticks, loosen both screws slightly.
See: How to Replace an Interior Door Handle