Is the door hard to close or open. Try to determine where the friction is occurring. There are two spots that take the prize most of the time: (1) The upper corner on the latch side of the door (the side with the door knob). (2) The bottom of the door, again on the latch side. Both of these conditions indicate that the door is sagging. Over time the weight of the door pulls on the top hinge and loosens it up. See Step Two below for instructions on tightening the top hinge and checking the door for alignment. This is a very common prehung interior door repair problem.
Is the door hard to move even when it is not going into the frame? Are the hinges squeaking or making noise? The hinges may be binding up. See Step Three for instructions on lubricating or replacing hinges.
Do you think the door is swelling? This is only going to be true if it is an all wood door and it is not sealed very well. Check the margins first before you start sanding or planing a door. See Step Five for instructions on sanding or planing a wood door.