The biggest causes of dead car batteries are

• Leaving your headlights turned on (DOH!)
• Breakdown of your alternator.
• Just having an old battery that's past its prime
• Not maintaining your battery properly with the correct water levels (for older maintenance batteries)

Of course some people get a bad starter, but the odds of that are low compared to the reasons mentioned above.

Here are the steps and some helpful notes:

1. Before connecting booster battery, make 100% sure that the swich is in the off position.
   

   

   Two cars ready for the jump start.
2. Make sure both cars are facing each other as seen in the image to the right.[2]
3. SAFETY FIRST : WEAR GOGGLES!!!  It's possible that the battery could explode if it's incorrectly connected or if there is something wrong with it.  If the battery is leaking, don't even attempt to jump start it. Just get a new one!
4. The two vehicles must not touch eachother!  This could create a ground connection through the exterior.
5. Clean off any corrosion that you see on the terminals.
6. Connect the red, positive jumper cable(+) to the positive terminal of the good battery.
7. Connect the red, positive jmper cable(+) to the dead battery.
8. Connect the black, negative jumper cable(-) to the negative terminal on the good battery. 
9. Connect the other end of the black, negative jumper cable(-) to something such as a bolt or braket on the engine block of the dead car.  Most of the metal parts inside the engine compartment are grounded.  Just don't connect it to a dirty or painted piece of metal or it won't work.  Find something shiny if possible.  When connecting this end you will probably see a small spark - this is normal.

NOTE: AVOID connecting the black (-) jumper cable to the negative terminal of dead battery.  This will reduce the chances that the battery will explode for some unforseen reason.

In some foreign vehicles (volvo, BMW...etc) the battery, as well as the battery terminals, can be 'cleverly' hidden.  Look at your owners manual if you have a hard time finding them.  If you search around the engine compartment you should be able to find something marked positve / red, and negative / black.

The cables are attached in this order to prevent sparking.  By connecting to the dead battery first it reduces the chances of the other end hitting something metal and creating a completed circuit before you are ready.
 

Happy car

If you are lucky it will start on the first try.  If it sounds like the engine is trying to crank but won't turn over completely, give it a few minutes to slightly charge the dead battery.  You can also rev the engine moderately on the good car.

Finally, after the vehicle has started, disconnect the cables in the reverse order of the connection.

Troubleshooting

• Make sure you have a good set of jumper cables.  If you have a multimeter you can check for continuity, or you can try getting another set of cables and swap them out for a test.
• Confirm that all your lights and appliances are turned off.
• Try giving it a few minutes to charge the battery before trying the jump start again.
• Make sure you clean off any corrosion on the terminals.
• If all else fails, call a tow truck or AAA to come out and jump start the car.  If you have emergency road side assistance included in your insurance you can call them to come jump your car or bring you to where you can get it fixed.  It's possible there is something wrong with your ignition system or alternator.

Note : The image of the happy car, originally created by David Hitchborne, is used under a Creative Commons License (CC BY-SA 2.0)