If your cell phone won't recognise your SIM card, or if it's displaying an error message of some kind, a good cleaning may be in order. Because the SIM card is a fairly sensitive piece of equipment, you should be sure to understand the proper methods before attempting to clean it.
Check to see if the SIM card has become loosened. Remove the back of your phone and its battery and set them aside. Remove the SIM card completely by sliding it out from underneath its metal bar. Place it back in face down and make sure it's securely in place. Wiggle it back and forth a little bit to make sure it's not loose.
Remove the SIM card if you're still getting an error message. Rub the tip of the Gold Guard Pen around the exposed gold part of the SIM card.
Remove the SIM card from the phone. Rub a dense white eraser along the exposed metal part of the SIM card. Make sure that the eraser is brand new, otherwise you could get the SIM card even dirtier.
Remove the back of your phone and the SIM card. Spray canned air into the back of the phone, removing all dust and dirt particles, especially those that could be stuck in the SIM card groove.