A squeaky door in your home is most often caused by dirty hinges. You could replace the hinges for around $15 per door, or spend around $10 and fix all the doors in your house.
Hinges squeak when there is friction between either the half of the hinge attached to the door and the half attached to the door jam, or between the hinge and the hinge pin. This instructable will only address the latter issue. If the hinge halves are rubbing against each other, it's probably because the top and bottom hinges aren't aligned and you probably are having much bigger problems with your door than just squeaking, like not closing properly.
Step 1: Tools
The tool list is pretty short. You will need:
Step 2: Removal
First we remove the hinge pin. Place the screwdriver under the lip of the top pin head and lightly tap it upwards. If the head of the pin is flush against the hinge, a chisel or putty knife might be needed to expose the lip. Alternatively, the bottom pin head is usually easy to remove and the screw driver can be inserted into the hinge and the pin can then be tapped upwards. Whatever method works, be sure to be careful not to mar the hinge with the screwdriver.
Step 3: Cleaning
Now that the pin is out, you can see how dirty it was. This dirt is what stopped the pin from rotating properly inside the hinge and made it squeak. Now wrap the pin in the steel wool and clear the dirt off. See how much came off? A little soap and water can be used as well, but be sure to let the pins completely dry before going on to the next step.
Step 4: Lubing
Now we relube the pins. Pour a little oil on a paper towel and generously coat the pin. Machine grade oils work best since they penetrate metal well, keep out dirt, and don't break down easily. I use the bar oil for my chainsaw. DO NOT use food grade oil like vegetable oil. And don't even try WD-40, it might be a great product, but it isn't a lubricant.
Step 5: Replacing
After the pins are all cleaned and re-lubed, place them back in the hinges. Tap them back into place with the hammer, but leave a gap big enough to fit your screwdriver into. The gap wont be noticeable, and it'll save you some time if you ever have to take the pins out again.