It's bound to happen sometime - the house keys are inside, you're outside, and the door is locked. Naturally, your first option should be a spare key, picking the lock, an unlocked window, prying the lock with a credit card, or even calling a locksmith, but if all of these fail, sometimes you've got to take a more drastic approach.

[edit] Steps

Stay calm. It's natural to be angry or upset, but it isn't going to help, and it's certainly not going to make your kick better. In fact, it increases the likelihood of injury. Before you knock down the door, make sure you've exhausted your other options. Is there any other way to get in? Have you considered breaking a small window that you can easily replace? Take stock of your situation, and if there's no other choice, go on to the next step.
Select an entrance that can most easily break upon impact and most cost-effectively be replaced.
Focus on the area just below the doorknob. This is where you will deliver the blow in order to apply maximum force. Take a breath or two, and prepare to strike.
Side Kick

Stand sideways a few feet away from the door, with your dominant foot facing toward it. With your dominant leg, side kick the area just below the doorknob quite hard. When executing this kick, you should not be "balanced" (as in the photos); rather, your center of mass should be in front of, not over, your supporting leg, giving you momentum. This should feel somewhat like you are "falling into" your target. Do not lean away from the kick, but keep your body upright as much as possible.
- The goal of this swift kick is to break the weakest part of the door which is usually the frame, the latch, or the lock. For most wooden doors without very large bolt locks, successfully using this technique will cause the trim around the door to break, allowing the door to open.
- It is advisable to hit with the sole or heel of the foot, rather than the edge. Since you are trying to transfer total kinetic energy, rather than to damage the area directly struck by your kick, increasing the impact area in this fashion is not detrimental to the effect, and helps reduce the risk of injury to your foot.
- It is also important to keep your base leg (the leg you are standing on during the kick) firmly on the ground to transfer the most energy into your kick. If you think about pushing from that heel it makes this easier.
Try once or twice more if it does not work the first time. However, you should not hammer away at a stubborn door too many times, because this may cause too much damage to the door (or your body). Try to find another way in, or just wait for help.

[edit] Alternate Method: Back Kick

Stand with your back to the door and kick like a donkey. This method offers a lot of force, more stability, and your foot lands flat just under the door knob.
Place your arms on the shoulders of a buddy standing in front of you (or use a table or anything that won't move and will support your weight) for further stability while you concentrate on landing your foot in the proper location.
Take a few practice "taps" to help prevent breaking your foot on the doorknob.

[edit] Alternate Method: Front Push Kick

Keep in mind that front kicking a door down will not offer as much force as a side or back kick, and increases the risk of injury dramatically, because the force used is not directly perpendicular to the door, and the toes of a foot may land on the knob or lock instead of the door, which may strain the ankle or break the tendons on the heel.
Stand a few feet away from the door, with your dominant foot placed in the back.
Use your dominant leg to front kick the area just below the doorknob quite hard (with a forward, pushing motion, not an upward swing). Do not use the ball of your foot to perform this strike; instead, use your heel. Be careful not to strike your foot on the doorknob.

[edit] Tips

You probably won't be able to kick it in if you are not kicking in the direction the door normally swings, so you'll want to check the swing of the door before you break your foot. When the door opens outward, removing the door hinges might be an easier way to get in. However, many commercial door hinges have theft proof hinge pins that do not allow them to be removed.
If you think you might need to use this skill at some point, practice it a few times on safe, easy targets. This will help ensure that you don't injure yourself when attempting the real thing.
It will help to exhale sharply or even yell when the kick lands. In martial arts this is called a kiai (pronounced "key-eye"). This tightens the core muscles of your trunk, and prevents your torso from absorbing forward motion (i.e., more energy is transferred to the door).
Do not jump when you kick the door. Jumping does not increase the power of your kick, unless you're jumping from a ladder or doing a spinning roundhouse kick, which is useless against a door. Jumping will give you less stability, decreasing the power of your kick and increasing the likelihood of injury.
Ramming the door with your shoulder can cause dislocations. Additionally, the impulse of the blow is lower, because your shoulder is soft and takes longer to deliver its force than your shoe, which is quite hard. Be sure to kick.
It may be easier and cheaper to break a window than the door. So if entry is the main goal, one should explore this option.
As an alternate method, you can use a hammer or sledge or other suitable pounding device to strike the area below the door knob. This is suggested for people who are not physically able to deliver a good kick. It may take a few more strikes since the impact will not be equivalent to a full-body kick.

Know the type of door you are opening and what kind of force it will require.

Hollow Core - These doors are usually inside doors with no insulation or security, need minimal force, and a screwdriver.
Solid Wood - These doors are made of hardwood and require an average amount of force and a crowbar.
Solid Core - The inside frame of this door is softwood with laminate on each side and a chipped or shaved wood core that require average force and a screw driver.
Metal Clad - These doors are softwood with a thin metal covering and require an average to above average amount of force and a crowbar.
Hollow Metal - These doors are much heavier than other doors, have reinforcing channel around the edges and the lock mounting area, and some have insulating material. They require maximum force and a crowbar.

[edit] Warnings

If you are not in good physical shape, do not use this method. It applies a great deal of force to the body, and should not be attempted by anyone with physical limitations.
Never straighten your leg all the way when doing this type of kick. Make sure your leg is still bent when your foot hits the target. Straightening your leg all the way can cause serious damage to the knee.
Either commit yourself to breaking open the door in one kick, or don't even attempt it. The force of your own kick against an object that does not give way could cause serious injury to your joints, including your knee and ankle. Make sure you are going to apply enough force to open the door in one kick. If you're uncertain of yourself, don't attempt this. Most wooden, interior doors of a house are easily opened in one kick, while exterior doors are not.
If you are in doubt as to whether you should try this, don't. If you want to build your confidence, consult a qualified martial arts teacher. You might find that it's good for more than just those times when you've locked yourself out of the house.
Naturally, this will damage your door and/or its frame. If you aren't willing to deal with that, you need to find another way in.
These instructions apply mainly to wooden doors without deadbolts. If there is a deadbolt, it takes a much harder impact and/or multiple strikes to smash the door frame. If the door and its frame are made of metal, you can forget about the whole thing, unless you are a martial arts expert
First, try to open the door with its own handle. It may already be open or unlocked.
If the door has glass in it, especially small panes of glass, break out the glass safely and unlock the door. It is cheaper to have glass replaced than an entire door and your home owners insurance may cover the replacement.