Clearance
If you try to fit a door into an opening the same size as the door, you're asking for trouble. Not only do doors grow and shrink along with changes in weather and humidity, their square shape causes them to bind if the opening is too small. As you start to swing a door outward, the opening has to accommodate the diagonal measurement of the door rather than its face. The thicker the door, the more of a problem this may present. Leaving clearance of 1/8 to 3/16 inch on each side of the door is usually adequate to alleviate these problems.
Conditions
Doors that rattle in February may not even fit into their jambs by August. This is because the material in the door absorbs water along with the humid air, causing the door to increase in width. This is much more pronounced in doors made of solid wood yet occurs to some extent in nearly all materials. If you're flexible about when you fit your doors, the best time is at the height of summer. In hot and humid weather, you can fit the door with quite a small clearance.
Hinges
For the hinges on a door to work properly, the door needs to have sufficient clearance and be oriented square with the door opening. If hinges are seated with their faces cocked in one direction or the other -- rather than being perpendicular to the opening -- they may not close completely before the door is closed. Forcing the door can damage the hinges. Avoid this problem by being careful to make the notches that the hinges sit in flat and consistent.
Solutions
If you have a door that's sticking, the easiest ways to solve the problem are to decrease the size of the door, increase the size of the opening or set the hinges more deeply into the side of the door. Of these three solutions the easiest is to decrease the size of the door by planing a bit of wood from the side opposite the hinges, using a simple, sharp hand plane. If you need to remove wood from the length of the door, you'll have to remove the knob plate first to avoid hitting it with the plane.