Take a cue from Eric Foreman and read how to break down a door before trying it for real. (And only if it's completely necessary, of course!) Maybe if he had read The Worst-Case Scenario Survival Handbook, he would have been successful.
1. Give the door a well-placed kick or two to the lock area to break it down.
Running at the door and slamming against it with your shoulder or body is not usually as effective as kicking with your foot. Your foot exerts more force than your shoulder, and you will be able to direct this force toward the area of the locking mechanism more succinctly with your foot.
Alternate Method (if you have a screwdriver)
Look on the front of the doorknob for a small hole or keyhole.
Most interior doors have what are called privacy sets. These locks are usually installed on bedrooms and bathrooms and can be locked from the inside when the door is shut, but have an emergency access hole in the center of the door handle which allows entry to the locking mechanism inside. Insert the screwdriver or probe into the handle and push the locking mechanism, or turn the mechanism to open the lock.
If you are trying to break down an exterior door, you will need more force. Exterior doors are of sturdier construction and are designed with security in mind, for obvious reasons. In general, you can expect to see two kinds of latches on outside doors: a passage or entry-lock set for latching and a dead-bolt lock for security. The passage set is used for keeping the door from swinging open and does not lock. The entry-lock set utilizes a dead latch and can be locked before closing the door.
1. Give the door several well-placed kicks at the point where the lock is mounted.
An exterior door usually takes several tries to break down this way, so keep at it.
Alternate Method (if you have a sturdy piece of steel)
Wrench or pry the lock off the door by inserting the tool between the lock and the door and prying back and forth.
Alternate Method (if you have a screwdriver, hammer, and awl)
Remove the pins from the hinges (if the door opens toward you) and then force the door open from the hinge side.
Get a screwdriver or an awl and a hammer. Place the awl or screwdriver underneath the hinge, with the pointy end touching the end of the bolt or screw. Using the hammer, strike the other end of the awl or screwdriver until the hinge comes out.